Every lift involves at least two terminals and may also have intermediate supporting towers. A bullwheel in each terminal redirects the rope, while sheaves (pulley assemblies) on the towers support the rope well above the ground. The number of towers is engineered based on the length and strength of the rope, worst case environmental conditions, and the type of terrain traversed. The bullwheel with the prime mover is called the "drive bullwheel"; the other is the "return bullwheel". Chairlifts are usually electrically powered, often with Diesel or gasoline engine backup, and sometimes a hand crank tertiary backup. Drive terminals can be located either at the top or the bottom of an installation; though the top-drive configuration is more efficient, practicalities of electric service might dictate bottom-drive.