Full Mortise Locks are inserted
in a hole with one side of the lock exposed.
Half Mortise Locks are inserted in half
of a hole, that is to say that when installed the lock has 2 sides exposed.
Flush Mount Locks are just mounted to
the surface.
In determining the measurements, the most important is where the keyhole
is in relation to the lock. ie...You may find a Half Mortise Lock that is
2" x 2" and fits the piece perfectly but the keyhole needs to
be located in the right spot within that 2" x 2" area.
When looking at a lock, you see two important things. First is what
I call the "Peg". It's the place where the key slides in and then
over the peg. Most old furniture keys have a hollow shaft which slips over
that peg. Second is the tongue, the part that slides in and out on the lock
when the key is turned. On most of our flush mount locks, the tongue slides
left or right which allows it to be universal. When selecting a flush mount
lock, if you measure your door from its' edge to the center of the keyhole.
Lets say its 1 inch. In order for the lock to fit, the measurement from
the peg to the side of the lock should be one inch also. It can be off a
little but should be close. If you were to choose a lock that is too small
then the tongue may not reach far enough to secure the door. The measurement
in length usually doesn't make any difference as the lock can be placed
anywhere up and down. This does not take into account any old screw holes.
It is not likely for you to find a lock to fit that perfectly. The best
you can hope is that the lock covers those old holes and fits properly.
I make it sound more difficult but it can be understood by carefully looking
at your particular use. | 
