Tune a guitar perfectly to a digital tuner, play a full open major chord, and something is still faintly off — a slow shimmer in the sound. The tuner isn't wrong, and neither is your ear. They're using two different definitions of "in tune," and the distance between them is a fixed, computable number. Two definitions of "in tune" A digital tuner uses equal temperament : the octave is divided into twelve exactly equal steps of 100 cents each (a cent is 1/100 of a semitone). T...