The phenomenon whereby a signal common to two lines opposite in polarity from one another gets cancelled at its destination. In balanced lines there is always a positive signal on one cable and a negative or opposite polarity signal on the other. The idea is that anything common to both wires will eventually get cancelled at the receiving end (as noted in WFTD Balanced). This CMR (Common Mode Rejection) is the principle that caused balanced lines to be invented in the first place. Nowadays it applies not only to various types of audio signals, but it is the underlying principle of balanced AC power systems as well. See also CMRR.