# Apartment key questions **URL:** \ **Category:** Factual Questions\ **Created:** [November 30, 2015, 1:14am UTC](https://boards.straightdope.com/t/apartment-key-questions/739002 "2015-11-30T01:14:38Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Laggard](https://avatars.discourse-cdn.com/v4/letter/l/9de053/32.png) [@Laggard](https://boards.straightdope.com/u/Laggard)\ **Post date:** [November 30, 2015, 1:14am UTC](https://boards.straightdope.com/t/apartment-key-questions/739002/1 "2015-11-30T01:14:38Z")
We live in an 8-plex. Each unit only has one key. These keys open the front gate, the common laundry room and the door to each unit. How is it that everyone’s key does not open all the units? ---
**Author:** ![feppytweed](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/feppytweed/32/4280_2.png) [@feppytweed](https://boards.straightdope.com/u/feppytweed)\ **Post date:** [November 30, 2015, 1:28am UTC](https://boards.straightdope.com/t/apartment-key-questions/739002/2 "2015-11-30T01:28:16Z")
Sounds like a kind of Master Key system. [Read more here.](http://home.howstuffworks.com/home-improvement/household-safety/security/question710.htm) ---
**Author:** ![Ethilrist](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ethilrist/32/4604_2.png) [@Ethilrist](https://boards.straightdope.com/u/Ethilrist)\ **Post date:** [November 30, 2015, 1:45am UTC](https://boards.straightdope.com/t/apartment-key-questions/739002/3 "2015-11-30T01:45:12Z")
Have you tried your key in one of the other apartments? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 30, 2015, 2:30am UTC](https://boards.straightdope.com/t/apartment-key-questions/739002/4 "2015-11-30T02:30:58Z")
The front door and the laundry room locks are missing a few tumblers (usually the ones on the keyring end). Basically, the common locks only look at the first several “digits”, and don’t look at the last ones. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [November 30, 2015, 3:37am UTC](https://boards.straightdope.com/t/apartment-key-questions/739002/5 "2015-11-30T03:37:15Z")
Master keys like this are common. In the long-gone days of my youth, my father owned a small janitor business, with contracts at several medium-size office buildings in the area. Every office had its own key (I would hope), and I suppose those individual keys would also all open the main doors into the lobby. But my father had a key that would open ALL the doors – it got him into the lobby and into all of the offices. Note that this seems to imply a scheme with the tumblers a bit more complicated than **Chronos** describes. One the one hand, you have _many_ keys (the individual office keys) that all open _one_ common door (the lobby) – that is the scenario that **Chronos** explains – but on the other hand, you also have _one_ key that opens _many_ doors. So you have a system with various overlapping subsets of keys opening various overlapping subsets of doors. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [November 30, 2015, 4:40am UTC](https://boards.straightdope.com/t/apartment-key-questions/739002/6 "2015-11-30T04:40:27Z")
On a 5 pin key lock system there are up to possible 1024 change keys that can be cut, less improper combinations. On a 6 Pin key lock system there are up to possible 4096 change keys that can be cut,less improper combinations . On a 6 pin system there will be, 1 Great-Great Grand works in all locks. 4 Great Grand Masters works in 1/4 of the locks. 16 Grand Masters works in 1/16 of the locks. 64 Grand Masters (or page masters) works in 1/64 of the locks. The key system is broken into 64 pages and on each page is 4 column masters 16 block masters 16 Row masters.