"The element of tuning is most important – one tunes each chord to to each note of the melody, not to the accuracy of a given key.  Thus each element of the chord not in the melody must be adjusted as if the melody pitch is the root of the chord.  In just intonation, the ratio of c to d in a c major scale is a slightly larger than the ratio of d to e.  Those ratios are slightly adjusted away from a scale in a compromise to make each chord “ring.”  When going around the circle of fifths one does not return to the starting pitch without making adjustments along the way!" - Antonio Pavio. Choralnet.org