"That ribbon" or "you-know-what ribbon". A ribbon worn around the chest area and tied to the upper arms by Hestia from Danmachi, so named because one look at where it's tied, and what it's holding up, is usually enough to determine its purpose (breast support). Can also refer to any similarly-styled ribbon used in parodies and crossovers. Tag underbust as applicable.
Ribbons that are merely tied below the breasts without supporting them do not qualify. They are most likely just dress ribbon and underbust.
