In article , John
writes
I just wondered why Yale locks were always fitted at eye level but mortise
locks were fitted half way up the door (traditionally)

The most common and vicious form of attack for a door[*] is kicking-in
so it makes sense to keep the strongest protection at waist height or
below. If using 2 reasonable locks as you describe, I'd have the mortice
at just above knee height and a deadlocking cylinder just below shoulder
height.

Having the mortice half way up isn't that great an idea as it leaves a
bit too much spring/flex in the lower part of the door making it a weak
point.

The top cylinder on its own is just to stop the wind blowing the door
open.
[*] Unless it's with a sledge hammer
--
fred
BBC3, ITV2/3/4, channels going to the DOGs