Cyclists have long worn a cycling cap to protect their heads from the sun's rays - this was more common in the pre-helmet era - absorb some sweat on hot days, and add some warmth in winter.

According to their manufacturer, Guardian Caps “bring a padded, soft-shell layer to the outside of the decades-old hard-shell helmet and reduce impact up to 33%.” The NFL reported that the ...

They have worked brilliantly in practice, so players who wish to protect themselves are now allowed to wear the helmet covers in games. What exactly are Guardian Caps, and how will the NFL's rules ...