MS is slightly different. MS is a stereo recording technique, used for a lot of things (an example is a stereo recording of a concert or a orchestra).
In MS there are 2 different microphones: The "M" microphonem wich a simple cardiod poitoing to the center of the "ensemble" you want to record
and the S, wich is a 8 figure microphone, pointing to both left and right sides of the "M" microphone and recording one of the sides in phase, and the other side out of phase. (M and S are coincident microphones)

While listening to a MS recording, you must have a "decoder": it sums both M and S signals (inverting the phase of the S the aquire both sides) resulting on a Stereo Recording.

I could explain a lot better the way that MS works, but I think I was quick and clear.

Luis Santos