The rudder is a crucial part of the plane because it guides the plane left and right. On an aircraft, the rudder is positioned on the rear fin that is on top of the body. This is a directional control surface, in other words, something on the plane that controls movement. This is along with the rudder-like elevators attached to the back fins (the flaps to the left and right of the rudder). The ailerons (attached to the wings) control the pitch and roll. The rudder is attached to the fin and this allows the pilot to control the yaw about the vertical axis. This changes the horizontal direction in which the nose is pointing, which changes the direction the plane is travelling.