A rotary combination lock is a lock commonly used to secure safes and as an unkeyed padlock mechanism. This type of locking mechanism consists of a single dial which must be rotated left and right in a certain combination in order to open the lock. Design and operation An externally-accessible dial is manipulated to release the shackle of a padlock or a lock bolt securing a door. The dial is connected to an internal mechanism, usually containing three separate wheels with notches, called gates. Each wheel must be aligned to allow a fence to drop into the gates, releasing the lever that holds the lock in place, allowing the lock to open. Generally, this is accomplished by moving the dial to three (or more) positions, usually denoted by numbers, in a specific sequence of alternating clockwise and anti-clockwise turns. The wheels are generally arranged in a stacked wheel pack sharing a single axis of rotation, and the individual rotary position of each wheel can be manipulated by turning the dial left and right. The dial is mechanically connected via its spindle to a cam that is equipped with a drive pin — a sort of nub — that can engage a corresponding small catch (known as a fly) on the side of the closest wheel facing the cam; the fly either can be fixed or move within a limited range. On the opposite face of that closest first wheel is another drive pin, which can engage a fly on the side of the second wheel (next-closest to the cam) that is facing the first wheel. Similarly, the second wheel has another drive pin on its opposite face that can engage a fly on the third wheel. Many combination locks have three wheels, but the lock may be equipped with additional wheels, each with a drive pin and fly, in a similar manner. The number of wheels in the mechanism determines the number of specific dial positions that must be entered to open the lock, so a three-sequence combination is required for a three-wheel lock. As the dial rotates, there will not be enough space for the drive pin on the cam to pass the fly on that face of the first wheel closest to the cam, and so the first wheel will begin to rotate with the dial once the drive pin on the cam engages the fly of the