(from Steven R.)
think it should be pointed out that it can be argued that we have been using the wrong term for Interlocks and we should be using Permissives.

Permissives:
Permissives hold a device at its passive state whilst the permissive condition remains active. Unlike interlock conditions a permissive condition, on becoming active, does not cause the device to change state to its passive condition. A permissive just prevents the startup.
Interlocks:
Interlocks perform two functions, they force a device to its passive state if it is active when the interlock condition occurs, and they also hold a device at its passive state whilst the interlock condition remains active.