'''Mechanical keys''' are small, metallic objects cut in a particular pattern, used to operate mechanical locks.
There are ten thousand 9 999 different key pattern types, serial numbered 0000 0001 to 9999; as such, a mechanical lock can normally only be opened by a key if their serial numbers match.
=== Keycards ===
'''Keycards''' are flat, thin pieces of plastic resembling a credit card, and containing a microchip with a hard-coded electronic signature. When read by an electronic lock, the electronic lock compares the keycard's signature with its own; if they match, it unlocks. Otherwise, it doesn't work.
There are exactly 65 535 different signatures, serial numbered using the hexadecimal system, from 0000 0001 to FFFF. Much like regular mechanical keys, electronic locks can only be opened by a keycard with a matching serial number.
=== Skeleton keys ===