A frame drum is a drum that has a drumhead width greater than its depth. Usually the single drumhead is made of rawhide or man-made materials. Shells are traditionally constructed of bent wood (rosewood, oak, ash etc.) scarf jointed together; plywood and man-made materials are also used. Some frame drums have mechanical tuning and on many the drumhead is stretched and tacked in place. It is the earliest skin drum known to have existed. Examples are found in many places and cultures. It has been suggested that they were also used to winnow grain.