Zi Collection, Middle Volume Radical: Ox (niú) Page 515, Entry 04 Pronounced pi (falling tone) "The coupling of wheels" — Book of Odes (Shijing) "The coupling of wheels" — "The Coupling of Wheels" (Shiming) explains that the coupling of wheels, like a secret interlocking, is located on the axle to secure the wheel and prevent it from moving forward or backward.