This security door bolt is ideal to secure the door at the top and bottom. It is morticed into the edge of the door and is undetectable from one side of the door. It is often used on internal doors to prevent intruder access to the hallway and the rest of the house should they have entered for example through a bedroom window. It can be fitted to external doors with key operation on the inside for added security. They are less obtrusive than surface mounted door bolts and are operated with a universal spline key.