The drum is a member of the percussion group of musical instruments, technically classified as the membranous. Drums consist of at least one membrane, called a drumhead or drum skin, that is stretched over a shell and struck, either directly with the player's hands, or with a drumstick, to produce sound. There is usually a "resonance head" on the underside of the drum. Other techniques have been used to cause drums to make sound, such as the thumb roll. Drums are the world's oldest and most ubiquitous musical instruments, and the basic design has remained virtually unchanged for thousands of years. Most drums are considered "untuned instruments", however many modern musicians are beginning to tune drums to songs; Terry Bozzio has constructed a kit using diatonic and chromatically tuned drums. A few types of drums such as timpani are always tuned to a certain pitch. Often, several drums are arranged together to create a drum kit.