Many doors use Pushbutton Combination Locks which require the user to enter a numeric sequence on a keypad to unlock the door. These are available in mechanical gear driven combination locks, as well as electronic combination locks. A mechanical combination can only have one combination at one time. An electronic combination lock can have several different combinations at one time, the number of combinations vary by manufacturer and series. The advantage to a combination lock is the ability to change the user code regularly. Most electronic combination locks rely on batteries or a power supply; however, there are now electronic combination locks available that do not require batteries.