Right page/head radical — simplified form of
頁, originally a kneeling person with an emphasized head (Kangxi #181). The indexing radical here. Marks
顿 as a head-related action: nodding, kowtowing, knocking the head against the ground (
顿首) — a literal pause-and-bow. Same radical anchors
颈 (neck),
颜 (face),
顶 (top),
项 (back of neck),
颠 (peak).