The title is a musical term describing a regular metric and harmonic theme upon which variations of melody and rhythm, voices or instruments are based. Here the proportions and positions of the colours - the blacks, whites and even the greys - are repeated in the same order throughout. Only the greys vary in moving from a light tone, close to the colour bands on both sides, to a deeper, nearly black shade just off-centre. This subtle movement and the displacement of balance give a surprising sense of depth to an otherwise flat space.