Dou Gomu come in larger sheets, but can be trimmed to a smaller size for a cleaner look! (Shown here) Like the rest of the shamisen, the highly polished surface of the dou (soundbox) is immensely gorgeous. The downside is that it can easily slide off your leg while you play! The dou gomu is a rubber pad which adheres to the corner of dou, allowing the dou to sit securely on your leg without slipping. The hiza gomu (coming soon!) is the classical version which doesn’t attach to the dou. Rather, it’s a larger pad which lays over your leg, and the shamisen sits on. The dou gomu is very convenient and is used by most all tsugaru shamisen players. The only downside of it is the rubber does hide some of the beautiful wood, and dries out after about five years and needs to be replaced. (It’s much more difficult to remove after it’s dry and crumbly, so better to replace them earlier than later!)
Again, you can still play shamisen without it, but the frustration of the dou slipping off your leg while you play is real.