The Gitzo GS3750D Panoramic Disc creates an independently-controlled panning axis underneath ballheads that don't have it, so you can move just the left-right axis without affecting the orientation of the ball itself. It has a built-in bubble level, and is sized to perfectly match the crown and top plate of Gitzo's Series 3 tripods.
The top has a fixed 1/4"-20 threaded screw, so that you could mount your camera directly atop the disc if desired. A panning axis such as this can also be used atop a leveling base for square format cameras or for use with a camera L-bracket, eliminating the need for a heavier, taller, and more complex ballhead or 3-way head.
3/8"-16 to 1/4"-20 Threaded Reducer Bushing
Gitzo GS3750D Panoramic Disc