Each side of the body must be synchronized and the two sides are also coupled. That is, the left and right wings and thus the left and right halteres always beat at the same frequency. However, the amplitude of the wingbeat does not always have to be the same on the left and right side. This is what allows the flies to turn and is accomplished using a gearbox, much like what you would find in an automobile. This gearbox can change the maximum amplitude of the wing movement and determine its speed of motion. The wings of flies even have a clutch structure at their base. The clutch moves between grooves in the gearbox, to engage and disengage the wing muscles and also modulate the wingbeat amplitude. When the amplitude of the left wing is less than the right, the fly will make a left turn. Even though haltere movement is controlled by separate muscles than the wings, because the wings are mechanically coupled with the halteres, changes in wingbeat frequency extend to the haltere-beat frequency as well, but haltere beat amplitude does not change.