Bruce Bartlett’s article on stereo microphone techniques describes 4 methods of capturing a sound source in stereo. The article focuses on recording ensembles, but the techniques he details can be used anywhere a stereo recording is desired.
One goal is accurate localization. That is, the reproduced instruments should appear in the same relative locations as they were in the live performance. When this is achieved, instruments in the center of the ensemble are reproduced midway between the two playback speakers. Instruments at the sides of the ensemble are reproduced from the left or right speaker. Instruments located half-way to one side are reproduced half-way to one side, and so on.