A tiltrotor is an aircraft which generates lift and propulsion by way of one or more powered rotors (sometimes called proprotors) mounted on rotating shafts or nacelles usually at the ends of a fixed wing. Almost all tiltrotors use a transverse rotor design, with a few exceptions that use other multirotor layouts. A tiltrotor aircraft differs from a tiltwing in that only the rotor pivots rather than the entire wing. This method trades off efficiency in vertical flight for efficiency in STOL/STOVL operations.