Whistler’s painting is characterized by a stringent composition based on an arrangement of colors, surfaces, lines, and atmosphere rather than on aiming for a depiction of reality.
In the summer of 1871 Whistler painted a portrait of his mother in his London studio. The painting was to have a tremendous impact on the art scene. Whistler arranged the pictorial elements in accordance with a stringent aesthetic that removed it from the realm of the real, thereby prefiguring the atmospheric scenes of Symbolism.
The title, Arrangement in Grey and Black: The Artist’s Mother, tells us that it is an arrangement first and a portrait second. Whistler used concepts from the realm of music, designating his paintings as e.g. nocturnes, symphonies, and arrangements. He emphasized composition over verisimilitude. Whistler insisted that painting is first and foremost an arrangement of... (http://www.smk.dk/en/visit-the-museum/exhibitions/past-exhibitions/2012/hammershoei-and-europe/arrangements/)