The timpani is often called a kettledrum because it is shaped like a kettle. The timpani has a large copper or fiberglass shell with a single drumhead. It also has a pedal mechanism which allows the musician to adjust the tension of the drumhead, thereby tuning the drum to different pitches. This makes the timpani the only drum which can produce definite musical notes. To produce the deep tone of the timpani, its drumheads are hit with mallets. Mallets are made of soft and hard felt or wood and will produce different tones on the timpani. Timpani are most often played in pairs or groups of four.