There are a number of key sizes / dimensions to measure when finding a replacement multi point door lock. We have sorted all of our available locks into specific sizes related to the lock backset and PZ centres. So for example, our 35mm / 92mm category will list all locks with a 35mm backset and 92mm PZ centre. The Backset dimension - measured from the front of the lock face plate to the centre of the spindle (follower) hole. The PZ dimension - measured from the centre of the spindle (follower) hole to the centre of the top diameter of the lock cylinder (barrel) hole. (If there are 2 spindles always measure from the top spindle hole). There a number of other measurements that are useful in finding your replacement door lock, please see our UPVC Door Lock Measuring & Identifying Guide.