Keyed Different (KD): Each lock has an independent cut. (Can still be incorporated into a custom keyed system).
Keyed Alike (KA): Group of two or more locks with the same key cut. (Can also be incorporated into a custom keyed system).
Builder Keyed (BK): [a.k.a. Protecto Keyed (PK) Construction Keyed (CK)]. Temporary builder key system used during the construction phase.
Master Keyed (MK): Simple two level system; a group of locks (KD, KA or both) that are operated by a change key (homeowner) and a single master key.