背子(はいし)haishi
Haishi
Similar to a modern-day vest, a haishi is a short upper garment worn by women. Its two, straight body panels are sewn together along the center back and under the arms at the side. A narrow strip runs around the neck and front opening to form a V collar (tarikubi). After 730, a penchant for Tang styles made such vests popular among aristocratic women.
61st Annual Exhibition of Shōsō-in Treasures, 2009