Stereo and mono

A wide mix sounds big on headphones, but much of the world listens in mono or near-mono: phone speakers, Bluetooth speakers, clubs. Anything that cancels in mono disappears there.

Phase correlation

+1 means the channels are identical (mono), 0 means unrelated, below 0 means parts cancel when summed. Most mixes sit between about +0.5 and +0.9.

Stereo width

The share of energy in the side (left minus right) signal. Bass should be close to 0%; pads, reverbs and doubles can be wide.