Stereo, short for Stereophonic, is a system of sound recording and reproduction which uses two synchronised channels, usually referred to as A (left) and B (right). There are several conventions for colour-coding analogue signals; the BBC adopted the nautical convention of Red for Port (Left) and Green for Starboard (Right), which can be confusing when interfacing with domestic wiring which follows the Consumer Electronics Association standard of red for right and white for left. A German convention uses red for right and yellow for left.
Digital audio uses the AES/EBU interface which carries both channels in one connection.
Mono (M) may be derived from a stereo signal by simply summing the left and right channels (A+B), but this would result in a signal level higher than the original channels, so it is usual to have M=1/2(A+B) i.e. the mono signal is 6dB lower than either left or right. The difference (S) between left and right is S=1/2(A-B).
A system aligned as above (referred to as M6) results in left, right and mono levels reading equally on meters i.e. A=B=M, and this is now standard practice for UK broadcasters. Early BBC experiments found that such an alignment did not give listening results which were as satisfactory as when the mono signal was reduced by only 3dB, and BBC Radio (and UK commercial radio) originally used the M3 alignment.
Stereo PPMs have two concentrically-mounted needles; the A-B version has a red needle for left and a green needle for right, and the M-S version has white for M and yellow for S.