On a stereo disk the two groove walls carry related but nevertheless independent signals, each wall undulating at 45° to the record surface. What happens to the groove from moment to moment with various combinations of left and right signal is illustrated in diagram 2.

With modulations on one groove wall only, whether left or right ([a] and [b]), it will be seen that a stylus perched firmly in the groove must move both laterally and vertically. In stereo recording and reproduction those sounds emanating from the centre of the stereo soundstage are represented by exactly equal signals in the left and right channels, and in accordance with an agreed convention such signals operate the disc cutter so that as one wall goes up the other comes down [c], resulting in purely lateral groove movement. The groove has constant width but meanders from side to side. This of course, is the same as on mono discs, which can be regarded from this point of view as stereo recordings of central sound sources only. The last condition depicted in [d] represents the case of a groove moving purely vertically, where the two signals are momentarily in total opposition or, in technical jargon, out-of-phase. If the two channels carry the same signal but in anti-phase, the groove bottom is `straight' but alternately narrows and widens; this forces the stylus to move up and down and so is called `vertical modulation'. It produces a diffuse sound with no defined position on the stereo soundstage.