One cardioid microphone and one bi-directional microphone in the same point and angled 90° creating a stereo image through a so called MS-matrix
MS Stereo means using 2 different microphones and a special matrix to create stereo spread and localisation. M means Mid mic and S means Side mic. Normally a cardioid (often hypercardioid or shotgun) microphone capsule is chosen for the Mid channel, but omnis are sometimes preferred to capture low frequency richness at distance. A bi-directional microphone (figure-of-eight-microphone) at the same point, angled at 90° is the Side channel.
The MS signal can not be monitored directly on a left-right system. The MS matrix uses the phase cues between the Mid and the Side microphone to produce a left-right signal suitable for a normal stereo system. Due to the presence of the centre microphone, this technique is well suited for stereo recordings where a good mono-compatibility is needed, and is extremely popular in broadcasting.
The stereo spread can be adjusted after recording the two microphones seperately with the MS matrix, by blending the M and S components in appropriate balance.
The easiest way to understand the MS matrix is by studying these two simple calculations:
Left channel=M + S
Right channel=M - S
The MS system will give directional information since the bi-directional microphone captures two directions and only puts out one signal with mutual opposite polarity. When sound approaches the MS arrray from the right, it enters the bi-directional Side mic in its phase inversed side. The matrix calculation says that Right output is M minus S. If you subtract a phase inversed signal from something it is like adding it. Mathematically speaking minus minus is plus. So, voila, sound approaching the MS array from the right side creates a signal in the right output of the matrix due to a simple subtraction.
For the left side, the sound meets the Side mic in its in-phase lobe and is simply added to the Mid signal.