Mid and Side (M&S)
This technique, also described in Blumlein's famous patent referred to above, is better known than
Blumlein Difference Technique (BDT) but not by much.
It also uses two microphones, one facing forward (the Mid
mic) and the other facing to the sides at right angles to the first mic (the Side
mic). The Mid mic can be any polar pattern but the side mic
must be a figure of 8. The output of the two mics is then fed into an M&S decoder. This converts M&S into standard left / right stereo by sum and differencing the raw M&S signals.
This technique has many advantages, but the two most important ones are the ability to easily vary the width of the sound stage by altering the level of the S channel in respect to the M channel and, because the microphones are coincident in space, you get perfect mono compatibility. The sound does not change between mono and stereo. If you record raw M&S on session, then you can adjust the width of your sound stage afterwards.
M&S is used mainly by film sound engineers as mono compatibility is an important issue, but perhaps is becoming less so in this Dolby Digital age. However, some Classical engineers have adopted it too.
Front view
Side view