There are two interlinked aspects to microphone placement: where the microphones are with respect to the room and the players, which controls the 'perspective' of the recording, and how the microphones are related to each other to generate a stereo image, which is the 'microphone technique'.
The various possibilities are summarised below, and the pages linked from the menu give some more detail.
Coincident techniques are those in which the microphones are placed as close as possible - often one above the other, so that they are truly coincident when related to the horizontal plane. These techniques vary according to the microphone directivity patterns chosen, and the angle between them.
Blumlein experimented with both the twin fig8 configuration (in fact, now generally called "Blumlein") and the MS arrangement. It is interesting to note that all possible coincident microphone techniques can be generated from the B-format signals generated by a soundfield microphone, which is a selling point for their manufacturers.
All coincident techniques clearly rely entirely on level differences to encode directional information; but the amount of out-of-phase information from the back lobes of highly directional microphone patterns can hugely influence the effect in playback.
Alternatively, the microphones may be placed apart, separated by a small or larger distance. The microphones may be either omnidirectional or (less often) directional. In this case, although there may may be level differences, especially with widely spaced microphones, the chief directional information recorded is time and phase differences.