A device for mounting two microphones on a single mic stand. They generally consist of a bar (sometimes adjustable in length) with a fixture to mount to a mic stand, and then two adjustable fixtures for microphones. Usually used for stereo recording, the stereo bar allows you the option of positioning the two microphones exactly as you wish to optimize the stereo image. Some engineers believe this approach introduces small timing differences into the recording due to the inherent imperfections of positioning the mics by hand, but it is a perfectly acceptable and time honored technique, especially if the microphones face outwards rather than inwards after you attach them to the bar and line them up. The timing differences that can occur stem from the fact that each microphone casts a sound shadow (literally, it physically gets in the way of the pickup pattern of the other) at high frequency, and if they face inwards this is likely to degrade the stereo image (particularly if the mics in question are physically large, such as AKG C414s, or Neuman U87s). If the mics face outwards, the sound shadow will fall on the rear of each microphone, where it is relatively insensitive (assuming the mics are set to cardioid, hypercardioid or supercardioid patterns) and will not cause imaging problems. Some stereo bars have detailed markings to aid the engineer in making fine adjustments to mix spacing and angle.
A typical studio application using a stereo bar would be to place mics directly above the drummer, with the microphones angled down towards the drums and outwards at approximately 90 degrees from each other. This gives good separation and minimizes phase problems.