The mechanical combination lock usually has three discs inside. The last disc is connected directly to the dial, and a notch on this disc rotates the second disc. A notch on the second disc in turn rotates the first disc. The lock is usually opened by rotating the dial clockwise to the first number, counterclockwise past the first number and to the second number, and clockwise to the third number (some locks use the reverse directions, and most safe locks require a return to 0 after entering the combination). This type of lock is popular on safes and padlocks. It is available for entry doors, but is not popular due to the amount of time required to enter the combination. The combination is usually set at the factory and can not be changed easily (if at all).