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Historically, units of measurement are often based on sizes of body parts. Probably the most obvious example would be a "foot" literally being based on the length of someone's (or everyone's, or anyone's) actual foot. Other examples include inch (length of a thumb, from tip to first joint), cubit (elbow to tip of middle finger), and span (from tip of thumb to tip of little finger, when spread as far as possible).

For most historical human populations that used these types of systems, this worked well enough for their day to day lives, despite the different sizes of body parts from one individual to the next. When that size variation was a problem, they could standardize, somewhat, by using the body part of a ruler/monarch/lord/etc. instead of just some random individual.

Enter the worldbuilding conundrum of populations that include both human and non-human races, some of which have distinctly differently sized body parts, or might even be missing a body part that another race has. The exact races present are not necessarily relevant to the question, but for illustrative purposes, I'll provide a couple quick examples. A lizard-human-hybrid type race would have a tail, that a human would not. A race of merfolk that are more fishlike than the standard trope might have webbing between their fingers or might not have fingers at all with fins or flippers in their place. Giants or dwarves/halfling-esque races might be mostly human shaped, but wildly out of proportion to humans in actual size.

In a world-building setting where at least 2, if not more, such races commonly and constantly interact, making body part lengths inappropriate as common units of measurement, what is the most likely basis for such a society to use as units of measurement instead?

EDIT

First, apparently I wasn't clear enough that I was looking for answers that did not include the use of body parts of the races, at all. As in: body parts are off limits as an answer. The question was meant to ask for alternatives to body parts, not variations on the use of body parts or how body part usage could be standardized.

Next, a note about the monarch and other variations of standardization, I understand that a monarch creating a standard is often how things were done, but this question was meant to be about what that monarch himself might have based his official standard on, if not his own body parts or someone else's. Sure, his official bar of metal could still be used as the defined standard, but what did he base the length of his official standard bar of metal on?

Answers that basically state "they'd all just agree or learn to convert": sure, but what would they use as a basis before they learned to agree on one or convert, if not body parts?

Assume the location is actual Earth, with the only difference being the additional races present, and assume ancient and/or medieval, as anything more specific shouldn't really affect the answers, since it's about what people from any of those time periods might use. In other words, what things on Earth, other than the race's body parts, would make a good basis for widespread common units of measure?

A commenter mentioned Barley Corns, and that is actually the only reference I could find in my research that was actually used historically, and this is exactly the kind of thing I was hoping for in answers. Other items in comments and answers that fit the spirit of the question include horsewidths, trout lengths, birch leave length, ant lengths, bananas, elephant tails, and narwhal tusk. Though horsewidths and barley corns are the only ones that I both know enough about and can confirm are sufficiently consistently sized and widespread.

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Measurement units have followed a fairly standard pattern.

Originally things are compared to something that both parties recognise. It could be a specific thing (the Washington monument) or a general thing (a banana). No one is really measuring anything accurately, so this is good enough.

Next people start to measure things against their own standard. This is usually merchants, who need to measure to set a price. They need something repeatable, so they use a particular stick for length. It might be similar to the length of a banana, because that's what people are used to, but other people may have chosen a different basis for their stick, and even if they didn't, their sticks will be different.

Next all the different measuring sticks are standardised by a local authority (king, merchant's guild, etc) so they can collect taxes. They either take an average or go with whatever the richest merchant was using. The same process can occur when two countries standardise - but normally a small country adopts the units of a larger neighbour.

Finally scientists define a more precise version of the standard.

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  • $\begingroup$ do have an example for the use of the length of a banana, because they vary in length by a huge degree. $\endgroup$ – John 2 days ago
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    $\begingroup$ @John - I'm referring to the Reddit meme "banana for scale". knowyourmeme.com/memes/banana-for-scale It's a real-life example of people using something common even though it's not a precise unit (as you point out). It can be used in a photo to indicate that a car is a model not a real car, or that a particular dog is very small, but once you need more accuracy, you need a better unit. $\endgroup$ – Robin Bennett 2 days ago
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    $\begingroup$ Note that merchants aren't the only people who care about measuring things. In particular, in an agricultural society, having standardized ways of measuring land can get pretty important. Especially if the society is situated on a floodplain (like that of the Nile in ancient Egypt, or of Tigris and Euphrates in Mesopotamia) where physical border markers can sometimes get washed away, or when someone gets the bright idea of levying taxes in proportion to cultivated area. $\endgroup$ – Ilmari Karonen 2 days ago
  • $\begingroup$ @IlmariKaronen but by that time they are already using a standardized unit of measure, whatever the "foot" is based on no longer matters. $\endgroup$ – John yesterday
  • $\begingroup$ @Ilmari - Thanks, I've added a note about kings standardising so they can collect taxes. $\endgroup$ – Robin Bennett yesterday
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You give your own answer!

When that size variation was a problem, they could standardize, somewhat, by using the body part of a ruler/monarch/lord/etc. instead of just some random individual.

There is some historic individual who has made a big impression on all of these involved races. Measurements reference body parts and famous exploits of this legendary explorer / philosopher / inventor / conqueror / lover.

This would be fun for a fiction because with each reference readers would wonder what all this individual got up to during his action-packed time. Maybe he even shows up late in the story.

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    $\begingroup$ This is generally how imperial measurements worked. They didn't just use any old "foot," they used the king's... $\endgroup$ – stix Aug 14 at 21:29
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    $\begingroup$ @stix Reminds me of Zork's bloit: "Defined as the distance the king's favorite pet could run in one hour" meaning it could vary wildly depending on the current sovereign. $\endgroup$ – aadv Aug 14 at 23:36
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    $\begingroup$ But often the king isn't handy to make the measurement. So copies of his/her foot are made and distributed, and voila... there's your standard measure. $\endgroup$ – Bohemian Aug 15 at 7:20
  • $\begingroup$ @stix The only reference to that I can find is the Roman pes Drusianus, but the length itself was already use before it was named after Nero Claudius Drusus (who was Governor of Gaul, but never Emperor - his Brother, Son, and Stepfather all were though!) The other 'standard measure' for a Foot was to grab 16 people at Random, and then average their foot length... $\endgroup$ – Chronocidal Aug 15 at 12:10
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    $\begingroup$ The question is about what could be used, instead of a body part, before it is standardized. Yes, they can and will standardize, but I'm looking for what could that standardization be based on, other than body related measurements. $\endgroup$ – Dalila Aug 15 at 13:50
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Well, for one, it's unlikely that having multiple races would affect using body parts for measurement; humans come in varying shapes and sizes after all, and that's why we moved to more objective forms of measurement as our technology allowed for it.

At most, you'd wind up with a situation where they say things like "that's an orc-foot long" or "that weighs a dwarf-pound."

However, I imagine just waving away your question isn't a very satisfying answer, so let's try and figure out a solution through world-building.

Creating a metric system requires a significant amount of technology and understanding of mathematics and the sciences, and even modern SI units are poorly defined for certain dimensions (the speed of light is, by definition, 299792458 m/s, but the meter is defined as the distance traveled by light in 1/299,792,458th of a second, so you can see the problem, the kilogram was defined by a 200 year old reference standard until only a few years ago, and its replacement, a silicon sphere, isn't without controversy).

However, since it's a fantasy world, you can invoke a bit of magic, handwaving, or both.

You could have a tree that inexplicably always grows a branch of a roughly identical length, or have a race of people that always reach the same height upon reaching adulthood. You could have a magical spell that's been bestowed upon the world by God of Measures, that allows for proper measuring.

Another possibility is, depending on how savy your people are, or how well they can measure time, would be for them to measure the difference between two shadows of a stick at different times on a specific day. For example, your measure of length might be something called a "springbow" which is defined as the distance between a shadow cast by a stick at the 1st bell and 4th bell on the day of Spring Equinox. Assuming that the time of bells is well-defined, presumably through a well-regulated clock, one would expect this distance to be identical on the Spring Equinox every year.

As such, you could even have a "Festival Of Measures" where the townsfolk gather to observe the measuring of the springbow, and reference measuring sticks are provided to all in attendance for free.

Even this has some problems though, as for the springbow to be identical, it has to be measured at exactly the same time in exactly the same place and must neglect the effects of procession. Worse, you have to know exactly where to measure on your shadows, and measure from that point every time. Procession is easy to hand wave away; you could say it's small or nonexistent for your world. The place is also easy to define. You could use a marker in some important city in the kingdom, perhaps the Royal Courtyard or so. The last question, where to measure on the shadow, is much trickier, and becomes a question of "who measures the measurer?" However, this is a problem we, with all of our technology, deal with today as well. Even though we can 'define' what we mean mathematically by a kilometer or a volt, measuring it is not as easy, and we're constantly revising our system of measurement as our technology allows for higher precision measuring of it.

So in your fantasy world case, perhaps you could use last-year's official springbow reference stick as a kind of "ritual of renewal." This will limit, but not eliminate the error in measurement, but would still leave you with measurements that far exceeded the precision of the ancients, plus it provides for a good plot device that can lead to interesting storylines (what if someone steals the previous year's springbow, or it gets lost, for example).

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    $\begingroup$ The meter is defined with respect to the speed of light simply because we can measure time very much more accurately that length. So the primary unit is the second, the speed of light is defined as a specific value, and in this way we can specify very very accurately the unit of length. $\endgroup$ – AlexP Aug 15 at 13:07
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    $\begingroup$ The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole - en.wikipedia.org/wiki/Metre. This rather supports your statement that "Creating a metric system requires a significant amount of technology" $\endgroup$ – Robin Bennett Aug 15 at 14:09
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    $\begingroup$ @AlexP I'm not sure what the relevance of your statement is? An accurate description of the meter requires significant technological and scientific understanding. To properly define the second, and thus a meter, you need an atomic clock, which is itself an incredibly complex expression of science and technology. You also need strong mathematical capabilities in order to express a value as small as 1/299792458, to say nothing of the understanding of chemistry and quantum physics needed to understand cesium's ground state transitions. All of this only reinforces my answer. $\endgroup$ – stix Aug 15 at 15:22
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    $\begingroup$ Sorry folks, but it doesn't need any form of "advanced technology" to create an accurate "international" system of units. It only requires a need to do it, which was "international" trade. Since trading involves physically carrying stuff around, carrying standard weights and measures as well is no big deal. For example, consistent standardized glass weights (made from glass because that is tamper-proof - you can't shave them down) have been found in archeological sites all over the Middle East, and their accuracy and consistency would be perfectly acceptable for most modern commercial uses. $\endgroup$ – alephzero Aug 16 at 9:40
  • $\begingroup$ "the kilogram was defined by a 200 year old reference standard until only a few years ago, and its replacement, a silicon sphere, isn't without controversy" - The kilogram was redefined just this year: en.wikipedia.org/wiki/… $\endgroup$ – Wolfgang 2 days ago
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We have used non-human body parts for measurement before. Roman roads were just broad enough to allow for two horses to go side by side on them.

If anything, though, the usage of body parts from onespecies might make the other feel misrepresented. This may lead to tensions which will hasten the adoption of some sort of universal, scientific measure. The revolution from which this arises may be called in your world le metric tensor.

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    $\begingroup$ Even barleycorns were a standard for a while, much like the official yard was a bar of metal held by the local lord or the king (on market days a central place held a copy for everyone to base their measurements off of)- measurements need not be their own anatomy, just something at hand that everyone local can agree upon. Since long-distance trade was not so heavily standardized or having much need of international consistent measures until recently, a local reference is all that is needed. $\endgroup$ – pluckedkiwi Aug 14 at 20:57
  • $\begingroup$ @pluckedkiwi Yes, barley corns were the only item I could find that was actually used historically, and not body part based. horse widths is probably the answer that best fits my question so far. $\endgroup$ – Dalila Aug 15 at 13:15
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    $\begingroup$ And for some other measurements: the stone, horsepower, poppyseed, furlong, league, bovate, virgate, carucate, grain, clove. Most of these are based on animal abilities. en.wikipedia.org/wiki/English_units $\endgroup$ – computercarguy Aug 15 at 19:40
  • $\begingroup$ There's also koku from Japan, which I've heard as the amount of rice an average person can eat in a year, but the Wiki doesn't back that up. en.wikipedia.org/wiki/Koku $\endgroup$ – computercarguy Aug 15 at 19:46
  • $\begingroup$ Note that the thing with the standardized roman roads is just a myth. Roman streets did not have a standardized width, though of course many were built to be wide enough for two horses - those of the local race, that is. Not that it is invalidating your answer in any way... $\endgroup$ – toolforger 2 days ago
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Measurements were originally derived from things aside from the human body.

The acre was originally the amount of land that could be plowed by a yoke of oxen in one day, and eventually became defined as a strip of land a furlong in length and a chain wide, with the furlong the length of a furrow said oxen could plow in one go before having to rest.

What's needed, initially, is something that is convenient for all the races in question, that is pretty standardized, and they agree on. For instance, suppose there's a widespread species of tree that, when mature, tends to have a thickness of around (what we'd call) a foot. So if you have one of those trees, some chopping yields you measuring sticks.

The single greatest issue is convenience. The units that stick around are the ones that can be used every day without needing excessive precision. A good example is the foot. Despite Canada officially using metric for decades now, the foot still sticks around because it's a unit of convenient size for everyday use. There's really no natural subdivision of the meter that works. On the other hand, litres are close enough to quarts, and thus to quarters of a gallon, that said switch worked out easily and quickly. Meters easily replaced yards (except in football) because they're both about the same size. But the good old foot, that's going to stick around for a while yet.

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  • $\begingroup$ "Despite Canada officially using metric for decades now, the foot still sticks around because it's a unit of convenient size for everyday use. There's really no natural subdivision of the meter that works." - I'm confused. What does "1 foot" accomplish that "30cm" doesn't? Do you mean that people actually use their feet to measure things? Or is it that 3 feet aren't close enough to 1 meter? $\endgroup$ – Ruther Rendommeleigh 2 days ago
  • $\begingroup$ @RutherRendommeleigh it's a lot easier to think and talk about a foot than 30 cm. $\endgroup$ – arp 2 days ago
  • $\begingroup$ @arp - That's the statement that I fail to understand. Maybe it's a cultural thing. I tend to use both. $\endgroup$ – Ruther Rendommeleigh 2 days ago
  • $\begingroup$ @RutherRendommeleigh, there's two things at work. First, we're next to the US and have a common market in many items, that they obviously dominate, so we have to know it. That's why sheets of plywood are 4x8 or 4x12, dimensional lumber is sold in lengths of 8, 10, 12, or 16 feet, and so on. But that also means we have ready-made references all around us, which makes it easy to make comparisons. A standard room has an 8 foot ceiling, a door is normally just over 6 and a half feet tall, and so on. $\endgroup$ – Keith Morrison 2 days ago
  • $\begingroup$ @ruther think of it this way. There are things you measure in km, in meters, in cm, in mm. Unless you're doing science you don't use Angstroms. There are things to be measured where a meter is too big and a cm is too small but a foot is natural and convenient. $\endgroup$ – arp 2 days ago
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Different races world most likely still use their own body parts among themselves. When interacting with other races they would learn to change between units.

If the races have been in contact with each other for a long time, it would be easier to use measurement units taken from elsewhere in their nature, from animals and plants common enough that different races all know them. Trout length, birch leaf length, three ant lengths and so forth.

A system similar to a metric system is obviously the best, but that was not the question.

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They could continue to use their own body parts, and simply be aware of conversions and other rules of thumb. If a human and a halfling are negotiating for some number of feet of cloth, they should establish which culture's "feet" are being used. (Or, maybe they don't and this is one way unscrupulous vendors try to swindle people in your universe.)

In pre-modern Earth it was common for well-established cultures to nonetheless have different units and different definitions of the same unit coexisting. Take the pous, the Classical Greek "foot", for instance. In Athens it was typically a hair under 300mm, but in Aegina it was about 10% larger.

Of course, this sort of untidiness leads to added difficulties in map-making, treaty drafting, trade, and taxation, so governments don't tend to like it very much. Among the accomplishments of most of your serious empire-builders, you'll find that they inevitably establish standard measures to be used throughout their empire. (I'm thinking here particularly of Qin Shi Huang, but there are many other examples.) Probably some of the "body" units in your story will fall out of favor as the cultures that promulgate them do the same, to be replaced by "standard" units based on those of the preeminent cultures.

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The reason people used body parts was its something everyone could reference. Given that's not the case with multiple fantasy races, then they'd likely use some other common reference. Perhaps they'd measure lengths in bananas. Or Elephant tails. How much is that rope? Its 2 coppers per Narwhal tusk.

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    $\begingroup$ this was what i was going to go with, just make up a fantasy fruit that grows with little deviation in length or size, "oh that'll be 5copper for a globbo's length of rope" to get to the bakery turn right in four streets, its then about 10 globbo trees down the road. $\endgroup$ – J.Doe Aug 15 at 14:03
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If you already have standard measurements for, e.g. time, you could base things off of that. E.g. a candle that is tall enough to burn in one hour is a candle-length. Sure, some one-hour candles might be taller than others, but the same is true for the body-part measurements that would otherwise have been used. If a certain type of candle has been around long enough to become a basic staple of modern life, you could expect its length to be relatively standardized. You could even mark smaller increments of time and length by having ruled candles with marks for every e.g. 5 minutes, and units of length could be tied to the amount of candle that burns in a given amount of time.

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Look at the Holy Roman Empire. Basically every city and principalia had its own units. This shows us two things concerning your question: Firstly that people could life and trade with different units of measurement and second that those units supposedly based on body parts lost their reference after a while. For example the Hessian Klafter was 2.5m, Considering that the Klafter is supposed to be the reach of a man's arms(similiar to a fathom) and therefore quite exactly the size of one, it seems that Hessians were giants back then.

I don't know exactly why you don't want to measure using body parts. But then the question becomes very broad. Basically everything that has a somehow uniform size could be used as a unit of measurement. And when societies grow larger and grow together they will start to standardize their units of measurement.

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    $\begingroup$ The reason I don't want to measure in body parts is because, in this scenario, body parts are not "uniform size". So saying something like "a hand-span" doesn't work because one race's hand might be only a few CM wide, by our measures, while another race might have hands half a meter wide ... or "5 paces" for one race might be covered in a single step by another so "paces" aren't uniform. Etc. $\endgroup$ – Dalila Aug 15 at 21:57
  • $\begingroup$ "when societies grow larger and grow together" +1 . OP describes a civilization utterly without racism in a situation ripe for speciesism. That, I simply cannot fathom. But "growth" is about money: whomever's got the money dictates the decree, and with their tiny little lizard hands, it's doubtful it would be them. - Forget measuring; how did y'all learn to get along and live happily ever after? The downtrodden will simply have to adopt that of their superiors. If that isn't the case, they've surpassed even our own civilization. #plutoisaplanet $\endgroup$ – Mazura yesterday
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Use circles. The angles would be consistent regardless of the size of the circle, especially if you make them concentric to evaluate them. Then use the distance the stars move in a particular time frame. Perform a few calculations and you can come up with latitude and longitude and from defining a unit of measure as the set fraction of the distance between latitude lines you can derive the equivalent of nautical miles.

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Why not use the size of the Earth itself? This is, after all, how real life got both the nautical mile (conceptually one minute of latitude, or 1/21 600 of the circumference) and the meter (1/40 000 000). It can be measured with good-enough accuracy using the knowledge available to Eratosthenes (276-195 BCE).

Of course, as technology improves, more accurate surveys can be taken, while the standardization definition of the length unit will likely stay the same for the sake of backwards-compatibility. This is how we arrived at our current situation where Earth's polar circumference is 40 007.863 km (error of 197 ppm from the conceptual 40 000 km) or 21 602.517 nautical miles (error of 117 ppm from the conceptual 21 600).

You will of course need to choose a convenient fraction of the circumference to use as your main length unit. And this is where having multiple sapient species makes things get complicated. For not only do you have to deal with orders of magnitude (e.g., a mouse would likely prefer a smaller unit than an elephant would), but you can't assume that everyone will be counting in base ten, which IRL comes from the human habit of counting on our ten figures. Some races will have different numbers of fingers, and some won't have fingers at all. So maybe you have one base-6 society and one base-10 society. Maybe they'll compromise on base 8 (their arithmetic mean) or base 30 (their lowest common multiple). In the former case, you might define a “foot” equal to 1/8^9 of the planet's circumference (298 mm), and in the latter case, you might have a unit that's 1/30^6 of the circumference (54.88 mm).

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We're in this exact situation on this planet right now:

Map of countries not using the imperial system

Within one part the planet, the one type of aliens use one system, whereas in the other part the other aliens use another system.

So the answer to your question is: Wherever the aliens do not talk to one another, they use their own system, but if the one alien tries to sell something to the other alien or wants to talk to the other alien in very precise measurement, they just relabel their products for the other alien's market because the customer is king and you want to serve your customer as best as you can! So there is no common system, unfortunately...

:-)

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The 'Problem' and the Answer

If you're disallowing body parts as the foundation unit, even if they're the body part of a particular individual, then that leaves other physical objects, and repeatable physical phenomena.

For example, one culture might use a {unit} defined as the length of the shadow of some famous monument, taken at local noon on the Equinox. This length will be identical to the shadow of a same-height object at any other location of the same Latitude, and once 'reference' {unit}s are created and shipped around, any town can create a durable monument which casts the same length shadow at local noon on the equinox, allowing them to create their own {unit} sticks locally.

Disclaimer

But, really, you're working on solving a non-issue in the first place. Even if a 'foot' was determined by the size of your own king's foot, those measures would vary from kingdom to kingdom, and conversion charts would be created by merchants who traveled among them, so they knew how to price their goods for the local market. (Otherwise, selling your textiles in a kingdom with a size 13 foot would be vastly less profitable than selling the same textiles in a kingdom with a size 6 foot.)

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You are chasing a non-issue. Variation among races doesn't matter.

By the time the variation of other races matter, your culture isn't using natural measurements anymore. handspans work fine even with a wide variety of races because by the time the difference in hands matter you are not using real handspans anymore.

First nothing in nature is consistent enough for precision measurement, They get used early then abandoned, the name sticks around the unt they no longer use the actual thing. examples include body parts, grain seeds or livestock body parts, feet were the most common just becasue the first thing that people are measuring is ground distances and feet are convenient. People are lazy they are not going to carry around things to measure with. seeds get used for volume because there is nothing convenient to measure volume with.

Of course for much of human history it did not matter, measurements were basically just tallies and eyeballed measurements. What a culture will pick is entirely situational, based on the local wildlife and population. Handspan, grains, feet, ect. works fine because you are either only trading with locals (debt based trade) in which case they can both see your hand and already have a culturally ingrained idea of what you are talking about, at this point measurements don't have a lot of use and when they do it is things that can be directly compared, from here to that tree, yay high, large vs small. Eyeball measurements are good enough nothing more precise is needed.

If you are trading with strangers measurements don't matter, trade is goods for goods that are both present at the same time, in which case unit measurement in pointless. Once you get to long distance trade you are already at the level that standardization begins, because languages are not shared in common. You are using direct comparison (scales, standing two things next to each other, ect.) just due to language barriers. Measurement does not have a lot of use until you start getting widespread writing, which is a long way away, and grows with standardization.

by the time you get to a culture that needs unit precision they have standardized, usually based on some kind of stone monument. It may have a name like handspan, but its based on the hand of a single statue or set of standardized statues there is minimal variation. It doesn't matter how big the hand of the guy you are trading with is.

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  • $\begingroup$ >This question is chasing a non-issue based on a misunderstand[ing] of how measurements [were] used historically. At the point a culture is developing its first units, variation of other races doesn't matter, the units are not coming into play. By the time it does matter they have already begun to standardize. - (too long; didn't read. This excerpt from chat is better wording) $\endgroup$ – Mazura yesterday
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There are some natural quantities that are fairly constant. If your society has a modern understanding of physics, it could use some of these constants.

$g = 9.807\ \rm{m}/\rm{s}^2$: A pendulum 1 meter in length swings at 0.5 Hz1. Given the number of seconds in a day, the monarch could figure out the right pendulum length. And, in one second, a dense object falls about 4.9 m. This is why movies scenes filmed on a tiny set need to be slowed down.

$g$ and $P = 1\ \rm{atm}$: A column of water forms a vacuum at about 10 m at 1 atm pressure2. 1 atm is also 760 mmHg3.

However, as you noticed, even if your monarch uses these definitions, the average person probably won't, unless your monarch gives out lots of rulers. And even modern rulers are only accurate within maybe 1% and change length with temperature.

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Lots of good answers here, but one point missing is the patterns of trade. Weights and measures start with trade. Initially you are going to have things like a trader using his own arm to estimate one yard. Where humans trade with humans this works fine. It also works fine when dwarves trade with dwarves: the fact that a human yard and a dwarf yard are different is not important.

Where life gets interesting is when dwaves and humans meet at the market. Modo Snorisscousin (dwarf) knows that he needs three yards of leather, but the product he needs is being sold by John Thatcher (human). You can write the dialog from there. Most likely Modo and John will compare arms and conclude that three dwarf yards is about two human yards, near enough for trading. All over the market similar conversations are taking place. Eventually the rough conversion factors get to be common knowledge.

Except that three dwarf yards is actually a bit more than two human yards, so if you are selling 60 dwarf-yards of rope its going to be more like 36 or 38 human-yards, so don't get short-changed by those crafty dwarves. Exact knowledge like this will be considered an important part of merchant lore, but not widely shared because it lends competitive advantage to those who hold it.

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When it comes to worldbuilding, these differences and variations SHOULD be a part of that world. Erasing them says just as much about the world as letting them be--it says something about who leads in trade, about differences in culture, about government and far, far more than you would think.

The takeaway is this: instead of asking how you can have uniform measurement, you should be asking why it would exist at all. In the real world where humans don't have this variation in size and body parts, we already have and historically have had vast variations in measurements. It's never been uniform. So if your world is similar enough to draw a comparison, it actually makes less sense for there to be one single standard without a compelling, in-world cultural reason for this to be true.

You're worried about inconsistency when in historical fact, units of measurement in early history have always been inconsistent. Units of measure varied from town to town in Medieval times. Standardized measures are something that is determined by a ruler or ruling class or who has power in trade, which much then enforces that.

The re-frame is, of course, what others have suggested--that the physical difference between the races means two different standards of measurement. Even today, on a global scale, there are two different major standards. And within industries, there are other standards still, sometimes vestiges leftover from a much, much earlier time (such as the measure of "grains" in gems.)

Making the basis something that is more consistent is difficult. Even IF you use an agricultural product (barley's consistency means that it was used as a measure of weight and length in past times) what agricultural product it will be might vary as much as body size and weight from race to race.

Grain actually has been used in weight quite a bit.

And barley HAS been used in length

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