In answer to a question at lostoverture.com, I said:
M/S = mid/side
You record the mid signal with a cardoid or similar and pan it up the middle.
You record the side signal with a figure 8.
You then clone the side signal and phase invert it and pan each track (the original and the phase inverted) up the sides.
If you play just the mid signal, you have mono.
If you add the sides to it, you add more “stereoness”, more room.
If you collapse the mix to mono, the side signals cancel and you are left with the mono.
You can make adjustments by varying the left and right signals vs. the middle, and vs. each other, and by panning them; it’s a phasing trick.
As well, using a omni for the mid, or the sides is a option, or you can use a hyper-cardoid for the mid, and use matching or different types of mic’s (ex., ribbon vs. condensor, etc.)
Trad M/S has you stack the mics vertically, so they are equidistant from the sound source in order to control the phase differences you get from distance; hence, my little stand is non-trad, not “accurate”, but still a EZ, useful effect.