Reins of a vault Meaning, Definition & Usage

Definitions
  1. (Arch.), the parts between the crown andd the spring or abutment, including, and having especial reference to, the loading or filling behind the shell of the vault. The reins are to a vault nearly what the haunches are to an arch, and when a vault gives way by thrusting outward, it is because its reins are not sufficiently filled up.

Webster 1913