Peter is correct in what he is saying, except that when you remove the posi screw from the metal strip (do NOT remove the smaller, usually torx head ones as these hold the lock-mech in) turn the key in the lock barrel to 5past/25to and withdraw the barrel. Then measure the offset - can be all different sizes from L/H edge to black/grey actuator then from R/H edge to same. A popular size is 45/45mm but can be many sizes depending on door(sash) thickness. Refitting is a reverse of this. Hope this helps. Kev@K.L.Plastics
Kev, October 2006