Whistler Nocturne Tonalism is the moment Western painting realized a picture could be made of pure tonal modulation in one chord of colour. Misted dusk-blue field, a single warm gas-lamp halo, a faint bridge silhouette, the butterfly monogram in the corner.

The discipline: one cool chord plus one warm point of light. Maximum four or five values. Negative space dominates sixty to eighty percent of every composition. Subject is almost incidental — the painting organizes itself like music.