The microphones are positioned far away from each other. The distance between the microphones depends on the size of the sound source. The rule is to keep a 3:1 ratio between the distance of the microphones from each other, and the distance of the microphones from the sound source. These techniques are not mono-compatible and are therefore used only in certain contexts.
The Decca Tree is a spaced microphone array most commonly used for orchestral recordings. It was originally developed as a sort of stereo A-B recording [AB technique ] method adding a center fill. It is the most commonly used distant technique. The technique was developed in the early 1950s and first commercially used in 1954 at Decca Records, to provide a strong stereo image.
Setup: a Decca Tree is sometimes setup using a triangular metal support in the shape of a "T". The "T" support originally measured about 2 meters wide by 1.5 meters deep and was very variable. The following pictures shows this basic setup:
More often you find three microphone stands, depending on the size of the venue and amount of spaciousness required.
The technique traditionally uses three omnidirectional microphones. Variations have been performed using a coincident pair, in X-Y, Mid/Side (M/S), or Blumlein positioning, in place of the center microphone.