A mortise lock comes in many shapes and sizes although the basic mechanics of the lock include a lock body, with a bolt, that is mounted into a door, window or piece of furniture and a fitting that accommodates the bolt and is fitted into the opposing frame. The door knob and keyhole for the lock are embedded in the lock body and a key is required to lock and unlock the door, window or piece of furniture with this type of locking device.
Insert the matching key into the mortise key hole. Insert the key all the way so that it cannot be pushed in the key hole any farther.
Turn the key counter-clockwise by one half to one full turn, depending on the model, to force the bolt to slide to the left from its locked position.
Listen closely to make sure the bolt retracts into the body of the lock from its fitting in the door frame. You'll hear a "click" when this occurs. If the bolt does not unlock, jiggle the key in the keyhole and try the previous step again.
Turn the door knob counter-clockwise, if applicable for that particular mortise lock fitting, or push down on the door handle for other models.
Older mortise locks can be more difficult to open versus new mortise-lock models.