The saxophone is a type of single-reed woodwind instrument with a conical body, usually made of brass. As with all single-reed instruments, sound is produced when a reed on a mouthpiece vibrates to produce a sound wave inside the instrument's body. The pitch is controlled by opening and closing holes in the body to change the effective length of the tube. The holes are closed by leather pads attached to keys operated by the player. Saxophones are made in various sizes and are almost always treated as transposing instruments. A person who plays the saxophone is called a saxophonist or saxist.