Signal correlation

Correlation measures the amount of similarity of a signal between its left and right channel. To make sense of a goniometers reading:

+1: 100% correlation - the signals are absolutely 'in-phase' (eg a dual mono setup with identical signals)
0: no meaningful phase correlation between the left and right channel (no cancellation when summed to mono either)
-1: 100% identical signals, but 180 degrees 'out-of-phase' between left and right