釋門歸敬儀 2.23

2.23

七、明頭面禮足者，正是拜首之正儀也。經律文中多云「頭面禮足」，或云「頂禮佛足」者，我所高者，頂也；彼所卑者，足也。以我所尊，敬彼所卑者，禮之極也。如俗中重尊友者，不斥其名字，相名為足下者，義類同也。又如天子、太子，有所稱謂，不敢及形，或稱「乘輿」、「車駕」，或云「陛下」、「殿下」，皆敬儀一也。

Seventh, explaining bowing the head and face at the feet. This is the precise and correct rite of bowing the head. Sūtras and Vinaya texts mostly say "bowing the head and face at the feet," or say "bowing the crown of one's head at the Buddha's feet." What is highest on oneself is the crown of the head; what is lowest on the other is the feet. Using what is most revered of oneself to respect what is lowest of the other is the utmost expression of etiquette. Just as in secular customs, when one honors and respects a friend, one does not speak their personal name directly but addresses them as "beneath your feet" (zuxia)—the underlying principle is identical. Furthermore, in the case of the Son of Heaven or the Crown Prince, when addressing them, one does not dare refer directly to their person, but says "Imperial Chariot" or "Imperial Carriage," or says "Beneath the Steps" (bixia) or "Beneath the Palace" (dianxia); these are all of the same ritual of respect.