The early years of the twentieth century saw tone clusters elevated to central roles in pioneering works by ragtime artists Jelly Roll Morton and Scott Joplin. In the 1910s, two classical avant-gardists, composer-pianists Leo Ornstein and Henry Cowell, were recognized as making the first extensive explorations of the tone cluster. During the same period, Charles Ives employed them in several compositions that were not publicly performed until the late 1920s or 1930s. Composers such as Béla Bartók and, later, Lou Harrison and Karlheinz Stockhausen became proponents of the tone cluster, which feature in the work of many 20th- and 21st-century classical composers. Tone clusters also play a significant role in the work of free jazz musicians such as Cecil Taylor and Matthew Shipp.