In the U.S., the term riding lawn mower today often is used to refer to mid or rear engined machines. Front-engined tractor layout machines designed primarily for cutting grass and light towing are called lawn tractors; and heavier duty tractors of the same overall size, often shaft driven, are called garden tractors. The primary differences between a lawn tractor and a garden tractor are the transmission torque handling capability, frame durability, the rear wheels (garden tractors almost always have multiple mounting bolts, while most lawn tractors have a single bolt or clip on the hub), and the ability to attach ground engaging equipment such as plows or disk-harrows. Craftsman, MTD, Snapper, and other major mowing equipment manufacturers use these terms.