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I was recently reading an medieval heroic fantasy book and I was disturbed by the usage of the metric system. Distances, heights were expressed in meters and it reminded me advanced civilization and French Revolution.

Maybe metric system is not the best unit system if we had to stay consistent in a medieval fantasy world. It sounds like an anachronism to me.

I'm building a medieval world which has sparse cities, villages and primitive commercial trades. What would be a realistic unit system?

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    $\begingroup$ The nice thing about using the metric system is that the audience will understand it unlike archaic units. $\endgroup$ – sphennings Apr 13 '17 at 13:10
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    $\begingroup$ Roman system: digits, palms and paces for length, pounds for weight, amphorae for capacity. Byzantine. Old French. Pick from the list. $\endgroup$ – AlexP Apr 13 '17 at 13:11
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    $\begingroup$ Many authors take the position that their characters are actually using their own language and idioms, you are just reading the best possible translation. Extreme examples include Tolkien and the "S. Morgenstern" of William Goldman. I myself extend this to measurements. My characters actually describe length in glurts, weights in fibbels, and so on, but I have converted these to feet and pounds for the convenience of my readers (and myself). $\endgroup$ – cobaltduck Apr 13 '17 at 15:02
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    $\begingroup$ I've always joked about measuring speed in furlongs per fortnight. This may be your big chance to actually do that! $\endgroup$ – T.E.D. Apr 13 '17 at 15:33
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    $\begingroup$ There's an argument to be made in favour of sticking with what your audience know rather than being rigorously authentic in-universe. For instance, it'd be ill-advised to write every dialog in elvish because that's what characters speak; your readers would probably not be amused. $\endgroup$ – AmiralPatate Apr 13 '17 at 16:29
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Back in the middle age each city had its own system of measures, usually based on the human body.

Inch, arm, palm, span, foot were used to measure distances, for example.

Normally in the market square there was some sort of reference sample for lenght and volume (at least), which would come in handy when foreign merchants came to sell their goods and for paying taxes in nature.

If you stick to those, it should sound realistic for those times.

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    $\begingroup$ And cubit! Don't forget the cubit! $\endgroup$ – AlexP Apr 13 '17 at 13:12
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    $\begingroup$ I once played a D&D game with a GM who had different units of length, currency, even time, for every kingdom or city state unless there was a strong trade between sites. It was hell just asking for directions or buying anything. I have ever since been highly supportive of SI unit standardization. Cultural differences are for art, not measurement! $\endgroup$ – SRM Apr 13 '17 at 14:04
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    $\begingroup$ Somewhere in a Euro museum is the architectural equivalent of the Rosetta Stone. It is a carving of different units of length from all over the Mediterranean. Replicas have been found at quarries and build sites. It allowed construction of many Greek temples. $\endgroup$ – SRM Apr 13 '17 at 14:07
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    $\begingroup$ @AlexP One issue that cubit has that palm and foot don't have is that few people may know offhand how big a cubit is. (Even if they've heard of the unit.) For things like "foot" and "pace" it's semi-obvious about how big they are, whereas "cubit" may have your readers searching for a dictionary. If you stick with units with analogy-based names (foot, barrel, etc.) even people who only know SI will still have a rough sense of how big the unit is. -- It's not that you can't use "cubit" as a unit, I'd just recommend calling it something like "a forearm" instead. $\endgroup$ – R.M. Apr 13 '17 at 15:28
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    $\begingroup$ One of the giant but unsung breakthroughs of the medieval age was the recognition that a grain of barley was the same size and weight regardless of where you were. As a result, the 'grain' became the first recognized inter-regional, let alone international standard measure. $\endgroup$ – Paul Smith Apr 13 '17 at 23:17
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You've already got a nice collection of links in the comments and in answers to get you started.

Measurements during this time were not consistent from place to place, however, traders did do conversions, letting buyers know how a general measurement which was used more widely, stacked up against the local measure.

There are many complaints based on measurements in the annals of law during this time because of the inconsistency. In fact, a person selling cloth might even have their own measuring stick (which might just be a single, flat stick) that they might use as a measure, telling customers that each of those is a unit, with the varying cost attached.

About 1200 or so because of the rize of Medieval fairs, more interest in consistent measurement, mandated by local rulers became more prevalent. So, at the Troyens Hot Fair in Champagne for example, they mandated that all merchants use a standard of measure that they imposed. The Champagne ell (2 feet 6 inches) was the standard and was checked against the iron bar wielded by the Keeper of the Fair. The fairs in that region were popular enough, with enough goods coming through, that this method of measurement spread to other regions in France and Belgium.

But this doesn't mean that an ell in France was the same as an ell in Germany or England. There were literally hundreds of thousands of measurement units, and in less regulated places this could even vary from shop to shop.

England, perhaps because of its isolation, kept the Medieval system the longest. America's current measurement system of pounds and feet and gallons--is actually closer to that old Medieval system than the Metric system is, at least in terms of what the measurements were.

While there were provincial measurement systems in England, the places that traded the most were the ones that set the standard. So, London would actually send out measures of bronze or brass to the provincial towns to basically let them know what the standard was. They would then "copy" those and send them out to other villages which wanted them. By copy, I mean that they would counterweight something else, such as sand in a container of equal measure. Unfortunately, these copies and copies of copies were not always accurate (because means of measurement were not always that good). So a pound in one place might not be quite the same in another. (Jokes could come from this--like a town known for their pound being short, if the word spreads, can become a saying-- people might know that a Jameston pound will always come up shorter than a London pound).

Standards were reissued in the hopes of keeping things consistent, but until Elizabeth I in 1588, a uniform system in England was not well imposed. (Okay, there was an effort in 1496 as well).

If you are starting from scratch, I would use the American system as the guide, because it's the closest thing we have in modern times to what the systems were like then, rather than the metric system.

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I've got an interesting view on this as a Canadian, but I'm thinking not only will you see multiple systems, you will see multiple mixed systems. The best unit system to represent medieval would be arbitrary, inconsistent, and vary from city to city and person to person.

If you ask a Canadian how much they weigh or how tall you are, most likely you'll get a pound and foot/inches answer. I buy my potatoes in 10 pound bags, price my meat out per pound, and I know my cars fuel efficiency in miles per gallon. Proximity to US tends to keep these terms alive. Ask me to measure a distance that's not my height, I'll answer in meters...I drive Km/h and buy litres of fuel...and in that very same grocery store, I buy deli meat in grams, beverages in litres, and (perhap most ironic considering it's sitting beside the 10lbs bag of potatoes) my fruits and veggies are priced per Kilo. Well except berries that are back to pint.

To go a bit further...my Grandma's recipe book continues several entrees that actively list 1lbs of butter, 2 cups of sugar, and 0.5 liters of oil. Mixed measuring units is a part of life here, and I'm sure you're medieval land would realistically see people struggling with multiple measurements. It can get to the point of absurd too...If you ask weather, my dad will tell you degrees F. Any other topic will be in Celcius.

The only consistent might come through in distances...though even then it's pretty arbitrary. If a town is a 30 minute horseback journey, that time (30 mins) can work as a valid distance measurement. Albeit this isn't accurate as speeds of travel aren't consistent, but it does function.

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  • $\begingroup$ @DavidPostill Ha. Poated this from my mobile and it looks like autocorrect got the best of me. Im confusing it with miles per schooner anyway...miles per frigate maybe? $\endgroup$ – Twelfth Apr 13 '17 at 21:33
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Beyond that, use measurements based on the human body. Hands high, feet high, arm's, thumb-length, paces, days (walking) journey. These seem archaic, yet modern people don't need to memorize strange units; they can grasp the distances by looking at their body.

These values will vary from place to place, and some places might use feet and others arm-spans and others paces.

Another option is to use yards, which are basically meters.

For things like time, if hours/minutes are too "modern", you could replace time with "bells" instead of hours, with each "bell" being some fraction (say 1/4) of a day-length (one bell at dawn, one at noon, and one half-way in between, similar for the afternoon) or 1/4 of a night-length (so 8 bells/day, thus 1 bell every 3 modern hours at the equinox, but it varies with the time of year).

Without lots of clocks, mearing time in hours/minutes makes little sense.

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Wikipedia has a list of many unit systems that were used in medieval times. Those were basically units based on a natural object, like a plant or a length of a foot for example, or based on a reference object (a rod of some sorts).

The first variant makes it easier to have it available for everyone, but the second variant is more precise (for the one who have the reference).

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  • $\begingroup$ I don't know why, but Dannish units sound really badass! $\endgroup$ – Bebs Apr 13 '17 at 13:25
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Some ancient empires were surprisingly good at enforcing a single measurement standard across their realm. Later local systems were often build upon such prior systems or were the result of increasingly deviating copies of copies of reference measures. Preexisting or imported systems were often adapted to fit the predominant one with simple integer ratios.

Length

The most general units of length are usually related to approximate measures of the human body.

  • Ell, inch: The width or length of fingers, hands and arms or the span between two of them is common in artisan and other manual work.
  • Foot, yard: The length of a foot (with shoe) or a (double) step is more likely in anything working in or on the ground (farming, military, architecture).
  • Mile, league: The distance that can be traveled either without rest (either on foot or on horseback) or within a standard time unit (part of a day) will often be the largest one defined. In naval cultures, it is more likely to be defined in relation to the circumference of the planet, e.g. 1 nautical mile = 1 equatorial minute of arc.

While different domains will prefer different units they are quite likely made to relate by simple integer ratios. The relations may look arbitrary, e.g. 12:3:22, because of this and sometimes two units will have a fractional relationship with each other because they relate to a common base by different primes, e.g. 1 palm = 3 inches = 4 fingers. Note that 1 foot is 12 inches less for the convenient factors (2, 3, 4, 6) but more because 3 + 4 + 5 = 12: a triangle with these sides has a square angle (9 + 16 = 25) and this is a useful property of measuring devices in construction.

It would be unusual but not unthinkable to have an actual consistent system, e.g. with generalized dozenal relationships between units.

Area

Small area measurements do not have a dedicated unit. People would specify them by the dimensions of edges or the radius.

Land area (i.e. hundreds of square meters) is measured for different purposes. Their historic relevance will determine the local prevalence of units.

  • Acre: agricultural workload. A popular unit is the area that can be plowed in a day or half-day with the technology available when it was defined. This may be normalized to the rectangle made by two length units, e.g. 1 rood = 1 furlong × 1 rod. Such units may depend on the local soil and topography.
  • Rood: agricultural resources. Another, smaller unit useful to Farmers is one that can be measured with simple tools and relates to the amount of either seed required for or crops harvested from this area. That means it will be aligned with a unit of weight or capacity, e.g. the area that needs one bushel of barley seed.
  • Hide: military taxation. For administrative purposes, especially in serfdom cultures, there will probably be a unit that is the area of land necessary to provide for one horseback knight or something similar. It may be a simple multiple of one of the agricultural units.

Volume

Wet and dry measures will probably have different, though possibly related units. Most of them will be derived from standardized measuring containers or casks, which may in turn be aligned with the respective weight (like 1 liter of water weighing 1 kilogram) at least for the base size (e.g. tun and ton). More often, the calibrated containers are specified by their inner dimensions. You can either have three length values for a cuboid or two for a cylinder: height and either radius, diameter or circumference of the base. π is probably assumed to equal 3+1/7.

Other capacity units are usually derived by halving and doubling the base iteratively. Some steps on this idealized scale may not have canonic names, e.g. there is no half-gallon measure in English. During the course of history the system may get further distorted for one reason or another, so occasionally there will be a factor of 3 or 5, rarely 7, between units.

  • Pint: The size of a drinking glass is a general purpose wet measure.
  • Gallon:
  • Bushel: The amount of harvest a single person can carry cash be a useful base in agricultural

Weight

Weight and mass are not distinguished.

Very often, small weights directly correspond to the amount of gold or silver (originally) minted into coins of the same name. A pound will usually be the maximum unit for this, while it is the minimum unit useful for other applications, e.g. in agriculture or mining.

Multiple subsystems may coexist, e.g. Troy vs. Avoirdupois, or just differ in subdivisions, e.g. Troy=Apothecary ounce but the former divided into 20 pennyweights and the latter into 8 drachms or 24 scruples.

Subdivisions are less likely to be binary than capacity measures are, i.e. prime factors 3 and 5 are more likely to occur.

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In medieval times, travel was not easy. Even if there was a standard, it would be difficult to share that standard. The natural order of things would be to make copies of copies, allowing error to creep in. Or unscrupulous sellers could shave a little off the measuring stick and sell less product for the same money.

Further, what's the point of standardizing? If you're happy to buy a bag of weasel dust for X quatloos, do you care if the bag contains 5 kilograms? Or do you care that it was about the same size as last time?

Finally, the expectation and need of day-to-day precision of medieval life was quite different than it is today.

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    $\begingroup$ Lots of people never travelled very far.It became important at the Fairs because you had merchants from all over, all of whom used different measures from their provinces and towns, selling the same sort of goods in one place. Consumers did not like that the inconsistency meant it was difficult to compare prices, and since the fairs imposed standards of quality (some of which was actually determined by weight) having a consistent measurement system became important. And the people who sold at the fair brought those measurements back home. $\endgroup$ – Erin Thursby Apr 13 '17 at 23:13
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    $\begingroup$ Yes, and as soon as they got there, they would begin changing. There were several "definitions" of an inch, for example. Imagine the derisive comments about a shorter inch in a rival town X. "Oh that might be 6 inches in RivalTown!" $\endgroup$ – Tony Ennis Apr 14 '17 at 2:39
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    $\begingroup$ Yep. And the opposite was true as well. Some merchants would use it as a marketing ploy (not that that they had such concept). Bragging that theirs is a generous inch here in town X or shop X compared to the standard. And of course making those bawdy jokes about rival towns! :) $\endgroup$ – Erin Thursby Apr 14 '17 at 12:33

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