The performance of keyboard tone clusters is widely considered an "extended technique"—large clusters require unusual playing methods often involving the fist, the flat of the hand, or the forearm. Thelonious Monk and Karlheinz Stockhausen each performed clusters with their elbows; Stockhausen developed a method for playing cluster glissandi with special gloves. Don Pullen would play moving clusters by rolling the backs of his hands over the keyboard. Boards of various dimension are sometimes employed, as in the "Concord" Sonata ( 1904–19) of Charles Ives; they can be weighted down to execute clusters of long duration. Several of Lou Harrison's scores call for the use of an "octave bar", crafted to facilitate high-speed keyboard cluster performance. Designed by Harrison with his partner William Colvig, the octave bar is