This is from Modern Recording Techniques by Huber and Runstein :
The XY technique is an intensity-dependent system; it uses only the cue of an amplitude to discriminate direction. With the XY technique, two microphones of the exact same type are placed with their grills as close together as possible (without touching) and facing at right angles to each other. The midpoint, or 45 degree angle, between the two mics is faced toward the musical instrument and the mic outputs are panned left and right. Even though the two mics are placed together, the stereo imaging is excellent - often better than when using the spaced pair method. There is the added advantage of having no appreciable phase problems due to the close proximity of the mics. The angles of the two mics may be changed to fit the occasion, with the preferred angles being from 90 degrees to 120 degrees. The generally accepted polar pattern is cardioid, although two crossed bi-directional mics can give excellent ambient results.
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