From the outset, it was considered an important safety feature that ships be guided though the lock chambers by electric locomotives, known as mules, running on the lock walls. These mules are used for side-to-side and braking control in the locks, which are very narrow. Forward motion into and through the locks is actually provided by the ship's engines and not the mules. With large ships, there are two mules on each side at the bow, and two each side at the stern—eight in total, allowing for precise control of the ship.