《资治通鉴·唐纪·唐纪三十》
外祖父母最尊贵，所以与姨母的丧服相同。姨母和舅父虽然同属一个等级，但丧服的轻重有差别。
外祖至尊，同于从母之服；姨、舅一等，服则轻重有殊。
External paternal grandparents are the most venerable, so the mourning attire for them is the same as for maternal aunts. Although maternal aunts and maternal uncles are both of the same degree, the severity of the mourning attire is different.