This is the typical type of key used on antique and antique-style furniture. The reason it is called a barrel key is because of the post-hole drilled into the bottom of the key. It is distinguished from a bit key, which is more commonly used on antique or antique-style door locks. Together, bit and barrel keys are sometimes called “skeleton keys.” It is not true that all such keys are interchangeable. Some locks that use barrel keys are quite complex; the key must be cut precisely in order to operate it.