Before you buy a deadlock work out whether you have a left- or right-handed door. To do this step outside and look at the door, if the hinges are on the left, it's a left-handed door and vice versa. Also, check the backset, the distance between the outside of the door and the centre of the doorknob, that is, how far the handle is set back from the door edge. The most common backset measures in Australia are either 60mm or 70mm. But check your door just to be sure. And don't forget to read the instructions before you start.