In the Nara Period, a piece of thin leather was folded and joined to the center of the back section, and the upper and lower ends that correspond to tops of the folds had a piece of hemp inserted to make a core, before being rounded off.
奈良時代には薄い皮を折り込んで裏面中央で合うようにし、折り山にあたる上下端は麻紐の芯を入れて丸みをつけるものが多い。