Mid/side (M/S) coding is the most common form of joint stereo: instead of storing left and right, the encoder stores a mid channel (the sum, L+R) and a side channel (the difference, L-R). For most material the two real channels are highly similar, so the side channel is small and compresses very efficiently, freeing bits that improve overall quality at no cost to the stereo image — the original L and R are recovered exactly by simple addition and subtraction. M/S is reversible and lossless in itself; the savings come from how little data the side channel needs. The same sum/difference idea appears in microphone technique and in mastering tools that process the centre and sides separately.
