An aircraft in flight is free to rotate in three dimensions: yaw, nose left or right about an axis running up and down; pitch, nose up or down about an axis running from wing to wing; and roll, rotation about an axis running from nose to tail. The axes are alternatively designated as vertical, lateral (or transverse), and longitudinal respectively. These axes move with the vehicle and rotate relative to the Earth along with the craft. These definitions were analogously applied to spacecraft when the first crewed spacecraft were designed in the late 1950s. These rotations are produced by torques (or moments) about the principal axes. On an aircraft, these are intentionally produced by means of moving control surfaces, which vary the distribution of the net aerodynamic force about the vehicle's center of gravity. Elevators (moving flaps on the horizontal tail) produce pitch, a rudder on the vertical tail produces yaw, and ailerons (flaps on the wings that move in opposing directions) produce roll. On a spacecraft, the movements are usually produced by a reaction control system consisting of small rocket thrusters used to apply asymmetrical thrust on the vehicle. Ship motions File:Aileron roll.gif|alt=Roll animation of a plane|Roll File:Aileron pitch.gif|alt=Pitch animation of a plane|Pitch File:Aileron yaw.gif|alt=Yaw animation of a plane|Yaw Normal axis, or yaw axis — an axis drawn from top to bottom, and perpendicular to the other two axes, parallel to the fuselage station. Transverse axis, lateral axis, or pitch axis — an axis running from the pilot's left to right in piloted aircraft, and parallel to the wings of a winged aircraft, parallel to the buttock line. Longitudinal axis, or roll axis — an axis drawn through the body of the vehicle from tail to nose in the normal direction of flight, or the direction the pilot faces, similar to a ship's waterline. Normally, these axes are represented by the letters X, Y and Z in order to compare them with some reference frame, usually named x, y, z.