Timpani, or kettledrums, are unique among most orchestral drums because they are tuned to specific pitches. In the early days, players had to manually turn 't-handles' around the rim of the drum to tighten or loosen the head, which was a slow and tedious process. In 1881, the 'Dresden' pedal system was introduced, which allowed the player to change the pitch of the drum instantly using their feet. The pedal is connected to a complex series of rods and levers inside the drum that pull on the 'counterhoop,' increasing or decreasing the tension of the drumhead. This innovation allowed composers to write melodic lines for the timpani and even glissandos (where the drum is struck and the pitch is changed while the note is still ringing). Modern timpani heads are typically made of plastic (mylar), though some professionals still prefer calfskin for its warmer, more traditional tone.