When the door/access (to a cell, for instance) is closed, the upper key stays trapped while the lower cylinder is empty. As the lower key is inserted and turned, the door/access can be opened. As long as it is open, this key will stay trapped. In order to continue with the interlocking sequence, the upper key needs to be released. The lower key won’t be able to turn and lock the door until the upper one is inserted and trapped.

Specifications

 Design