In Beds for people who practise group marriage I describe how marriage works on the world of Thirdrock: in essence, these people practise polyamorous group marriage, with a marriage consisting of at least one man, at least one woman, and up to two (or very occasionally and temporarily three) other men or women, for a total of four (or occasionally, for a limited time, five) people.

Because of the rather Victorian-era mores of the society, there are places that only married people may visit. In order to ensure that unmarried people, or people who are married, but not to one-another may not enter, entry may only be gained through the use of keys that symbolise the marriage.

These keys would be designed so that the male and female keys were different but complementary, so that when used together, any pair of complementary keys could release a lock intended to bar access to those couples who are not married to each other... or who are not married at all.

For example, Alice, Bob, Carol and Dave are married to each other. Ella, Fred, Gertie and Horace are married to each other. Alice or Carol could open the lock with Bob or Dave present, but Alice could not open the lock with Carol, nor Bob with Dave, and likewise with the other married group. Nor could Alice or Carol open the lock with Fred or Horace, and neither could Bob or Dave open the lock with Ella or Gertie, since while each person is married, they are not married to each other.

Additionally, these keys must also be unique, so that each person can have a private area that only they can unlock, and no-one else, even if they are married.

Finally, there must be locks that any one person in a particular marriage could open, but no-one from another marriage could open.

Finally, all male clergy are considered to be married to all female clergy, yet each clergy member must also be able to operate only their own private locks with their keys.

The tech level could be considered to be mid-20th century, and the locks are mechanical.

How could these keys and the different locks be designed in order to accomplish these different tasks?

  • $\begingroup$ Divorce must be a really, really intresting logistics problem. What happens to the keys then? $\endgroup$ Commented Dec 4, 2020 at 4:00
  • $\begingroup$ @JustinThymetheSecond The person(s) divorcing from their marriage surrender their key. $\endgroup$
    – Monty Wild
    Commented Dec 4, 2020 at 4:06
  • $\begingroup$ How many people are on this venture, total? Even with regular keys, there are only so many different variations, such that there are perhaps hundreds of exactly the same key, only the chances of your key fiting any particular lock are very slim, but possible. $\endgroup$ Commented Dec 4, 2020 at 4:06
  • $\begingroup$ But do they divorce ALL people in the marriage? You love one mate, but hate the other? And what happens to their private lock? $\endgroup$ Commented Dec 4, 2020 at 4:07
  • $\begingroup$ Victorian morality, so one woman can be seen with one man at one time, but two women alone together and two men alone together have no public marital standing. You don't cover the condition of Alice at the door with both Bob and Dave. Can the "third wheel" enter? (Let's ignore keys for a sec and focus on the public morality.) $\endgroup$
    – JBH
    Commented Dec 4, 2020 at 4:07

2 Answers 2


This is solvable mechanically

So fun fact, your key design is actually stricter than what's currently on your front door. A standard door look has about 200,000 unique keys, any town with more than 200,000 buildings there are duplicate keys in existence. That aside, let's give a primer on how locks work:

With locks designed lock so:
enter image description here

The lock is reading particular bits of information from the key - you need a key on the corresponding depth so that the slit between red and blue line up allowing the key to rotate. If you don't care about a particular section of the key, just don't put pins in that section.

You can accept multiple values for a pin position by putting a 3rd or subsequent pin in the shaft, eg like so:

enter image description here

You need 1 pin for unique Id within a marriage (person 1 - 5, encoded as height above baseline as 0,1,2,3, or 4), and the remaining pins for the marriage ID. If you give each marriage an 4-pin ID (in base 5), encode male keys as the ID verbatim, and female keys as the inverse of the ID (each pin is the opposite height), you can get complimentary keys, with the last pin making each key unique.

For a 5 pin tumbler lock that only leaves 4 pins for marriage ID. With complimentary pin positions that only leaves 312 polys. So you'll need to add additional pins depending on the number of polycles. 6 pins will give 1500 polys. 7 pins will give ~7800.

The 1 pin for unique ID within the marriage needs to be the "first" one, the one closest to hand when inserted, the marriage Id are the remaining pins closest to the tip. This allows the pins to be read mechanically.

  • Private rooms are possible because every key is unique. Private locks check all pins match the required value, just like your standard front door lock.
  • Marriage specific rooms (Eg the polys home) ignore the member-in-marriage pin by leaving out the final pin. They have the split pin in each shaft to ensure that both male and female keys work.
  • Clergy is just assigned a special marriage ID. All clergy is married to all clergy.
  • The "married partner" rooms will have 2 locks, one for a male key, one for a female key, they're complimentary locks. Since the complimentary keys add together they can be inserted in either lock.

Eg, encoding each key as the height above the baseline for each pin position:

  • Alice 012345
  • Bob 143210
  • Carol 212345
  • Dave 343210
  • Ella 033221
  • Fred 122334
  • Gertie 233221
  • Horace 322334

Note that:

  • the final 5 values for some pairs add up to 55555. That's how we check for married but opposite gender.
  • Each key is unique. That's how each person gets their unique room.
  • Each poly has the last 5 values equal to one of 2 values, that's how we let polys into their own poly-specific rooms.
  • 000000 is not a valid marriage ID - it's given to police (or cleaners) as a master key.

Checking that two keys inserted have the same marriage ID but opposite gender (do the first 5 pins add up to 55555) is accomplished by putting pins in common shafts, like so:

enter image description here

enter image description here

  • If only a single key is inserted, it will go all the way in, but only that key can turn, releasing only one lock of 2.
  • If two keys of the same gender are attempted to be inserted, the blue pins will block the least inserted key at the second click.
  • Two keys with different marriage IDs will not be complementary, so the second key inserted will not advance.
  • Two complimentary keys inserted will shift the blue bars back and forth as they advance, and when fully in, they can both turn. Note that the keys need to be inserted in sync, representing the equality of married members to each other. If one gets more than 1 pin in advance of the other it will be held up until the other key catches up.

Socially - I don't think this will work.

As an aside - Every poly I've been in has shared resources, and shared access to the same spaces. We all have duplicates of each others keys. We can drive each others cars and get into each others homes. We can get into each others investment properties and know each others passwords.

This is just for practical reasons, we help each other out as required. But I couldn't keep a private key from my partners if I wanted to - they could easily take and copy it when my guard was down. And my guard is always down around the people I love.

If you offered my poly to pay for the locksmithing services to set this up we'd decline. Mutual trust has benefits, especially with property maintenance and round the house chores. Plus the odd surprise morning cuddles.

Your poly families are likely to be similar - they trust the people they love so allow access to their private spaces.

  • $\begingroup$ So that it isn't necessary to insert both keys into the complementary locks simultaneously, how would you work that? $\endgroup$
    – Monty Wild
    Commented Dec 4, 2020 at 9:17
  • $\begingroup$ @MontyWild I have no ideas currently on how that could be accomplished without compromising security. If I think of something I will edit the answer, but for the moment this should meet all the requirements of the question. $\endgroup$
    – Ash
    Commented Dec 4, 2020 at 10:45
  • $\begingroup$ Could it work if you have one keyhole, requiring a male and female key? The inside of the door is like a vault door, only opening by your suggested "both add each barb up to a certain value". That way you can only turn if both keys are inserted (would also work with seperate holes probably) $\endgroup$
    – Trioxidane
    Commented Dec 4, 2020 at 13:52
  • $\begingroup$ On your social note: It makes great sense, but you seem to assume their relationships are held together by things like joy, spontaneity, deep love and mutual vulnerability. What you write makes sense for us modern folks who enter/leave relationships at any time based on "how it feels", and have long discussions with our partners bout "boundaries" and the like. How much of it would hold in a controlled, buttoned up society where courtship is carried out according to strict rules, and where decency and public appearance is everything? Not saying you're wrong, I'm just curious. $\endgroup$
    – EdvinW
    Commented Dec 5, 2020 at 23:22

The Ministry of Marital Accessibility is here to help!

And it's a good thing, too! Because someone must be responsible for policing all of the locks that have been delivered to the sundry establishments that serve the needs of our clientèle polyamoureuse!

  • The Ministry is responsible for creating, maintaining, delivering, and installing the locks designed for legally-bound polyamorous marriages.

  • The Ministry is also responsible for creating, duplicating, renting (this is important), and monitoring all keys and key fragments.

  • Finally, the Ministry is duly authorized by Her Majesty the Queen to police and defend the use of both locks and keys. A very special suite of cellule de prison solitaire is maintained just west and a little below the Tower of London specifically for this purpose.

The Ministry has developed several kinds of locks to assist services and vendors.

  1. For the less discriminating provider, a two-key system that is universal male and female. Males are allowed key A and females are allowed key B. ixnay ethay appingsway ofyay eyskay, lest we introduce you to our courteous geôlier. Any Key A + Key B inserted into the dual-tumbler lock and turned simultaneously will ensure expeditious entry!

  2. For the more discerning provider, a single-key system requiring keys in parts is advised. Seen below, when combined properly, the two key parts become one key capable of actuating a dual-level tumbler system. Same rules apply as with #1. Image courtesy HistoricalLocks.com.

enter image description here

All this must seem mundane to the the layman who believe he or she understands keys! Most obviously any woman holding the female key of type #1 and any male holding the male key of type #1 can enter an establishment protected by such a lock! Yet, due to its lack of complexity and, therefore, lack of cost, the solution is well loved by services that cater to the individual in a marital relationship! We've learned not to judge nor to ask too many questions.

But for the truly service-oriented vendor, lock type #2 is all the rage! This is due to a Royal Patent that protects the method used to bring the two parts of the key together! Our unique solution allows only those keys from the same group to be bound together — a key-within-a-key! Our solution has a great number of combinations, allowing us to serve any number of blissfully married groups.

These serialized keys are issued to the first couple bound in Holy Matrimony and they are registered to them as le groupe parfaitement marié. As members join or leave the group, the other keys in the six-key set (three female, three male) are issued with the restriction of no more than five may be issued at a time. Thus, it is only possible for a male and a female from a legally joined marital group to combine key parts to form a complete key that will permit access to the establishment. Failing to return keys as parts or all of the union dissolve has... shall we say... conséquences.

It is with great delight that the Ministry of Marital Accessibility strives to find perfection with the privacy all parties desire!

Just to be clear about solution #2. It allows a guaranteed difference between male and female keys, but by "keying" together to two halves according to marital group, it forbids a female from group A to join their key with a male from group B. The interlock between halves is as important as the combined key itself. Consequently, the humorous Ministry is actually a fundamental and indispensable part of the solution. Someone with governing authority must ensure that Group A's keys are never delivered to Group B.

If you need an idea for how the two halves would come together, imagine a common bike lock key like the one below. One half of the key has a round "key" on its side, a small cylinder that's been notched similar to the tube shown in the image. For the sake of the obvious, let's call that the "male" key. The "female" key would be the lock side. The two halves are brought together, perhaps at a 90° angle, and then turned until parallel. In this way, no differing group keys could be combined. Image courtesy Westway Electric Supply.

enter image description here

And just to address a comment by Monty on the question... rather than the Ministry, each establishment could simply provide keys like this, guaranteeing unique keys per-establishment and per-group. However, I suspect a robust key-copying service will ensue without some form of governing authority. After all, unless we invoke 21st century biometrics, the key has no way of knowing who's holding it.

  • 1
    $\begingroup$ So... we have keys of type #2, where within a marriage, males have pattern X, and females pattern X'. But how can these keys also be used individually to open group locks, e.g. the family house, and personal locks, e.g. an individual room within the house that no others may enter? $\endgroup$
    – Monty Wild
    Commented Dec 4, 2020 at 5:33
  • 1
    $\begingroup$ The joining of keys is an interesting idea, but result in a situation where examination and alteration of the keys would in effect allow the easy creation of male and female master keys. What I had in mind was to use the lock to determine if the two keys were complementary, and somehow have the keys serve all three purposes. That wouldn't make key creation impossible, but would make it more difficult. $\endgroup$
    – Monty Wild
    Commented Dec 4, 2020 at 6:49

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