If a player wanted to wear a Guardian Cap during a game, the league would prohibit it - NBC Sports
Ravens Embrace the Funny-Looking Guardian Helmets - Sports Illustrated Baltimore Ravens News, Analysis and More
NFL executive hints that Guardian Caps could eventually be worn in games, along with position-specific helmets
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
What are the padded helmets players are wearing at NFL training camps?
Arizona Cardinals defensive end J.J. Watt not fan of the new league-mandated Guardian Caps on helmets
Guardian cap' to prevent head injuries mandated for practices, CFL says
Guardian Caps Guardian Sports
NFL training camps: Guardian Cap players are wearing over helmets, why
Guardian Cap debuts during minicamps in effort to reduce avoidable head contact
Guardian Caps: NFL requires use of helmet cover at training camps to reduce head injuries
NFL CMO Dr. Allen Sills explains importance of mandated guardian caps during training camp