In his 1917 piece "The Tides of Manaunaun", Cowell introduced a new notation for tone clusters on the piano and other keyboard instruments. In this notation, only the top and bottom notes of a cluster, connected by a single line or a pair of lines, are represented. This developed into the solid-bar style seen in the image on the right. Here, the first chord—stretching two octaves from D2 to D4—is a diatonic (so-called white-note) cluster, indicated by the natural sign below the staff. The second is a pentatonic (so-called black-note) cluster, indicated by the flat sign; a sharp sign would be required if the notes showing the limit of the cluster were spelled as sharps. A chromatic cluster—black and white keys together—is shown in this method by a solid bar with no sign at all. In scoring the large, dense clusters of the solo organ work "Volumina" in the early 1960s, György Ligeti, using graphical notation, blocked in whole sections of the keyboard.