Nowadays, the chin straps on football helmets cover the wearer's chin, but that was not so in the past. Football chin straps didn't cover the chin until Riddell introduced them with their plastic helmets in 1940. Before then, straps that went under the jaw, resting near the neck, so why in tarnation did they call them chin straps?
As best as I can tell, we adopted the chin strap term from military caps and hats with straps that went under the jaw, though they were commonly worn on or above the chin, especially in ceremonial settings. We still see this use in military settings and in the marching band uniforms that copied the military’s shako-style headgear.

Just as football players are penalized today for not keeping their chin straps properly snapped, a disorderly or missing chin strap was once a gaolable offense, though being drunk and AWOL didn't help either.
But back to football...
The soft canvas and leather helmets that arrived around 1900 had jaw straps comprised of strings that tied or straps that cinched the head harness in place.
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