Cases of word usage

1) (a method of, key to) secret writing (cipher)

2) (to make) a short, light sound (like that of a key turning in a lock) (click)

3) a device used to fasten a door, lid etc that needs a key to work it; a position of hair that naturally hangs or clings together (lock)

4) a key which opens a number of different locks (passkey)

5) to cause (two or more people, things) to change places; to put (a piece of music) into another key (transpose)

6) to use a key to open a lock on (a door, cupboard etc) (unlock)