The base part of the thumb has a bump (tsurumakura) and the form of tsurumakura is a very important part, since it affects shooting and the movement of a flying arrow.
親指根部分に弦を掛ける段差（弦枕）があり、弦枕の形状は射の運行、矢の飛び方に関わる極めて重要な部分。