If a player wanted to wear a Guardian Cap during a game, the league would prohibit it - NBC Sports
As NFL mandates Guardian caps in practice, area high school football teams have been using them for years, Sports
Guardian Caps helping to keep players safe
What Pros Wear: Guardian Caps What are Those Helmet Covers Being Worn in Practice? - What Pros Wear
What are NFL players wearing on their helmets at training camp and how do they work? - The Mirror US
What Pros Wear: Guardian Caps What are Those Helmet Covers Being Worn in Practice? - What Pros Wear
NFL players experimenting with 'Guardian Caps' to prevent head injuries
NFL CMO Dr. Allen Sills explains importance of mandated guardian caps during training camp
Guardian Caps: Why some NFL players are wearing them in training camps
Arizona Cardinals defensive end J.J. Watt not fan of the new league-mandated Guardian Caps on helmets