A strap or rein attached to a horse's harness that prevents the horse from lowering its head, used to control the horse's posture and movement.
From 'check' (to control or restrict) and 'rein' (a strap for controlling a horse). This equestrian term has been used since at least the 17th century when horse control was essential.
Checking reins actually became controversial because horse welfare advocates argue they're uncomfortable for the animal—fashion (keeping a horse's head held high) conflicted with the horse's natural comfort.
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