What is the difference between a rim lock and a mortise lock? The rim lock is a surface mount lock - the entire lock can be seen because it is mounted on the surface of the door. A mortise lock is mounted inside the door and the lock itself cannot be seen. A mortise lock is accompanied by trim which is mounted on the surface of the door. Trim can be plates, rosettes and key escutcheons, and it can be made out of many materials such as bronze, brass, iron, steel, wood, & even porcelain or gutta percha.