What does the rudder primarily control on an aircraft?

The rudder primarily controls yaw, which refers to the left and right movement of the aircraft's nose around its vertical axis. When the pilot inputs a command to the rudder, it affects the direction in which the aircraft is pointed. This capability is essential for maintaining coordinated flight, particularly during turns, as it helps to counteract the adverse yaw that occurs when a plane turns. By adjusting the rudder, pilots can keep the aircraft flying straight and level while maneuvering, making it a crucial control surface for directional stability. Understanding this function is vital for pilots, as it plays a significant role in aircraft handling and navigation.