The Mid-Side miking technique requires two different types of microphone polar patterns: Cardioid, and Figure-8. This is probably one of the best and most accurate stereo miking methods you can use. The problem with this is it must be decoded into stereo. Encoding and Decoding is very simple to alter the spread after the recording using a DAW:
1. Record the mid mic on track 1; record the side mic on track 2.
2. Copy or clone track 2 to track 3. Be sure the waveforms are aligned.
3. Pan track 2 hard left; pan track 3 hard right.
4. Reverse the polarity of track 3 or use an “invert polarity” plug-in.
5. Group tracks 2 and 3 so their faders move together.
6. To change the stereo spread, vary the levels of tracks 2 and 3 relative to track 1.
As mentioned this mic placement technique requires a Cardioid Mic that is positioned in front of and facing the instrument. This mic is the "MID" signal and captures in MONO. A figure-8 mic (the SIDE) is also used in this configuration. It is sensitive to sounds coming from the front and the rear of the mic. You'll position this mic right below that of the Cardioid mic with the front of the mic facing 90 degrees to the left. This is what gives you the stereo width and is used to capture sounds from the left and right sides of the stage/area. Depending upon the mic itself will determine what actual position the figure-8 mic is to be placed under the Cardioid.