Schneier on Security: Physical Access Control

Very nice description of security logic applicable to software (not all that powerful for fences as commenters point out....

Schneier on Security: Physical Access Control: "In Los Angeles, the 'HOLLYWOOD' sign is protected by a fence and a locked gate. Because several different agencies need access to the sign for various purposes, the chain locking the gate is formed by several locks linked together. Each of the agencies has the key to its own lock, and not the key to any of the others. Of course, anyone who can open one of the locks can open the gate.

This is a nice example of a multiple-user access-control system. It's simple, and it works. You can also make it as complicated as you want, with different locks in parallel and in series."