《礼记·丧大记》
作为敛衣的袍子必须外加罩衣，不能单用袍子；有上衣则必须有下裳，这叫做一套。
袍必有表，不禅，衣必有裳，谓之一称。
The robe worn as mourning clothes had to have a cover, and it could not be worn by itself; if there was an upper garment, there had to be a lower garment, and this made a set.