HOW TO MEASURE A LOCKCASE/GEARBOX

A - Handle/spindle centre
The distance from the centre of the keyhole to the centre of the handle/spindle. In this example there are two possible spindle points.
B - Lockcase length
The distance from the top to the bottom of the lockcase which sits inside the door itself.
C - Backset
The distance from the middle of the spindles to the front face of the lock.
D - Faceplate length
The distance from the top to bottom of the faceplate which sits flush on the door.
E - Lockcase depth
The distance from the faceplate to the back of the lockcase.

F - Faceplate width
The horizontal width of the faceplate that sits flush to the door.
E - Lockcase width
The horizontal width of the lockcase that is inserted into the void in the door.