This Alpha Dual-Gang 50K Linear potentiometer is a two-in-one potentiometer stack, for independent stereo control. It's an audio/log taper 50K potentiometer on each half, with one shared knob for both. It's smooth and easy to turn, but not so loose that it will shift on its own. We like this one because the legs are 0.2" apart with pin-points, so you can plug it into a perfboard. 

Folks love Alpha pots for their reliability and consistency - and we agree! They make great low-cost, high-quality potentiometers

This potentiometer has a round (not T18 or flattened) shaft that is nominally 6mm diameter (we measured 6.3mm with our calipers)

Once you're done prototyping, you can drill a hole into your project box and mount the potentiometer using the included washer and hex nut.