Hourglass drums are a sub-category of membranophone, or drum, characterized by an hourglass shape. They are also known as waisted drums. Drumheads are attached by laces, which may be squeezed during a performance to alter the pitch. The category also includes pellet drums such as the damaru, although not all pellet drums are hourglass shaped (such as the Korean do, nodo, noedo, and yeongdo, which are barrel shaped).