KTD - Keyed To Differ
A single Cylinder with an Individual Key pattern.
KA - Keyed Alike
Any number of Cylinders all opened by the Same Key pattern.
MK - Master Keyed System
Any number of Cylinders each with an Individual Key pattern, a single Master Key opens all Cylinders in the system.
GMK - Grand Master Key System
Any number of Cylinders each with an Individual Key pattern, a single Master Key opens a defined group of Cylinders but not other defined groups. A Grand Master Key opens all Cylinder in all groups.
CK Construction Key
The Construction key is used during installation, allowing workmen full access to all areas. The main key is then used to recode the Cylinder and to automatically exclude the Construction Key.