This distinction is not strict, as many North American players use the German setup and vice versa. Throughout their education, timpanists are trained as percussionists, and they learn to play all instruments of the percussion family along with timpani. However, when appointed to a principal timpani chair in a professional ensemble, a timpanist is not normally required to play any other instruments. In his book "Anatomy of the Orchestra", Norman Del Mar writes that the timpanist is "king of his own province", and that "a good timpanist really does set the standard of the whole orchestra." A qualified member of the percussion section sometimes doubles as associate timpanist, performing in repertoire requiring multiple timpanists and filling in for the principal timpanist when required. Among the professionals who have been highly regarded for their virtuosity and impact on the development of the timpani in the 20th century are Saul Goodman, Hans Schnellar, Fred Hinger, and Cloyd Duff. A few solo concertos have been written for timpani, and are for timpani and orchestral accompaniment. The 18th-century composer Johann Fischer wrote a symphony for eight timpani and orchestra, which requires the solo timpanist to play eight drums simultaneously. Rough contemporaries Georg Druschetzky and Johann Melchior Molter also wrote pieces for timpani and orchestra. Throughout the 19th century and much of the 20th, there were few new timpani concertos. In 1983, William Kraft, principal timpanist of the Los Angeles Philharmonic, composed his "Concerto for Timpani and Orchestra", which won second prize in the Kennedy Center Friedheim Awards. There have been other timpani concertos, notably, Philip Glass, considered one of the most influential composers of the late 20th century, wrote a double concerto at the behest of soloist Jonathan Haas titled "Concerto Fantasy for Two Timpanists and Orchestra", which features its soloists playing nine drums a piece. For general playing, a timpanist will beat the head approximately in from the edge. Beating at this spot produces the round, resonant sound commonly associated with timpani. A timpani roll (most commonly signaled in a score by ) is executed by striking the timpani at varying velocities; the speed of the strokes are determined by the pitch of the drum, with higher pitched timpani requiring a quicker roll than timpani tuned to a lower pitch.