Many different mic setups exist for location stereo recording.

Coincident Pair - Two mics arranged (typically) one above the other, so that sound waves reach both capsules at the same time. The following three techniques are coincident pair techniques:

XY - Coincident cardioids at 90 degrees

MS - Mid/Side. Use a single mic, which may be anything from omni to hypercardioid, facing forward (mid) and a figure-eight facing to the left. Remember that the back lobe of a figure-eight is _out of phase by 180 degrees_ from the front. When you add mid plus side, you get a left-pointing pickup. When you subtract mid from side (mid plus inverted side), you get a right-pointing pickup. If the mid mic is cardioid, the resulting left and right signals are cardioid at 90 degrees. Theoretically the result is the same as XY.

Blumlein - Coincident figure-eight mics at 90 degrees

Semi-coincident - Two mics angled to encompass the sound stage but also spaced between 6 and about 48 inches apart to add time-of-arrival/phase differences to the amplitude differences caused by the directional pattern.

The following techniques are semi-coincident:

ORTF - Office de Radio-Television Francaise - two cardioids angled 110 degrees, spaced 170 mm.

NOS - Nederlandsche Omroep Stichting - two cardioids angled 90 degrees, spaced 300 mm apart.

AB - Generally two spaced cardioids facing forward to slightly angled apart.