As found on some basses, these blend potentiometers are perfect for blending between two pickups.
They function as essentially two potentiometers at once, as the top is twisted, the two potentiometers sweep simultaneously from one end of their paths to the other. They are most commonly used by connecting the different pickups to the opposite ends of each of the 'internal' potentiometers - so that as the top is twisted, the output of each pickup is indirectly proportional to each other. Or as the name suggests, it has full output from each pickup at either end, and the sweep acts to 'blend' them together in whatever amount you wish.
Body diameter: 16mm
Shaft length: 18mm
Supplied with washer and nut.