FM radio: https://en.wikipedia.org/wiki/FM_broadcasting
> left and right, but with opposite phase
The classic mid-side mic technique uses two capsules. The first (middle) is a cardioid or omnidirectionsl facing the subject, the latter (side) is a figure-8 turned 90 degrees. The capsules should be as close to each other as possible to avoid low-frequency phase errors. The signals are converted to L+R via a simple Mid/Side network:
L = 0.707 * (M+S), R = 0.707 * (M-S)
which is where "opposite phase" comes into play.