The basic timpani drum consists of a drumhead stretched across the opening of a bowl typically made of copper or, in less expensive models, fiberglass and sometimes aluminum. In the Sachs–Hornbostel classification, the timpani are thus considered membranophones. The drumhead is affixed to a hoop (also called afleshhoop), which in turn is held onto the bowl by a counterhoop, which is then held by means of a number of tuning screws called tension rods placed regularly around the circumference. The head's tension can be adjusted by loosening or tightening the rods. Most timpani have six to eight tension rods.