An interlocked door is one of the most secure methods of access control available to site security managers. In its simplest form, an interlocking system is composed of two doors electronically connected so one cannot open until the other has closed. This prevents either door from being opened by hand. The second type of interlocking system allows for multiple-code access. Here, each door has a magnetic card reader that communicates with a controller which in turn talks to the building alarm system or some other security system. Only cards bearing appropriate codes can open the door. This type of system can be as simple as a push button on each door which activates their corresponding readers or it can be more complex such as a keypad on one door and a fingerprint scanner on another. In general, electronic interlocks are the most secure method of access control available because they prevent manual opening of doors. Mechanical locks can be forced open, but electronic interlocks cannot. They are also very efficient - requiring only electricity to open or close them - compared to traditional door locks which require effort to operate. Electronic interlocks may be used instead of or in addition to physical keys for additional security. They can also reduce costs by reducing the need for staff time to manage keys and lockers. Interlocking systems can be found in high security facilities including banks, casinos, and military bases.