NFL training camps: Guardian Cap players are wearing over helmets, why
NFL executive hints that Guardian Caps could eventually be worn in games, along with position-specific helmets
What Pros Wear: Guardian Caps What are Those Helmet Covers Being Worn in Practice? - What Pros Wear
Guardian Caps NFL: What player positions are required to wear them?
Why are NFL players wearing padding on helmets at training camp?
If a player wanted to wear a Guardian Cap during a game, the league would prohibit it - NBC Sports
Guardian Caps: Why some NFL players are wearing them in training camps
NFL players don Guardian Caps, designed to protect from concussion
NFL players experimenting with 'Guardian Caps' to prevent head injuries
What are Glacier Peak players wearing on their helmets?