In 1922, composer Dane Rudhyar, a friend of Cowell's, declared approvingly that the development of the tone cluster "imperilled existence" of "the musical unit, the note." While that threat was not to be realized, clusters began to appear in the works of a growing number of composers. Already, Aaron Copland had written his "Three Moods" (aka "Trois Esquisses"; 1920–21) for piano—its name an apparent homage to a piece of Leo Ornstein's—which includes a triple-forte cluster. The most renowned composer to be directly inspired by Cowell's demonstrations of his tone cluster pieces was Béla Bartók, who requested Cowell's permission to employ the method. Bartók's First Piano Concerto, Piano Sonata, and the [https://www.youtube.com/watch?v=0xU-p9wmSqg&t=6m40s "Night Music" from the "Out of Doors" suite (all 1926), his first significant works after three years in which he produced little, extensively feature tone clusters.