EXPLAINER: NFL using helmet caps for player safety, research
What's With Those Padded Caps NFL Players Are Wearing In Training Camp?
Guardian Caps: Why some NFL players are wearing them in training camps
Funky-Looking Guardian Caps Rejected by NFL for Regulation Play, OK'd for Practice - Core77
Puffy, spongy Guardian Caps could help curb NFL head injuries - The Washington Post
What are Guardian Caps? NFL's latest padded helmets for added safety
NFL training camps: Guardian Cap players are wearing over helmets, why
Guardian Cap Use a Concern for Watt, Saleh Early in Training Camp - Sports Illustrated
Coverage of the NFL in Guardian Caps
Number (sports) - Wikipedia
Guardian Caps Guardian Sports
Dallas Cowboys - Wikipedia
Funky-Looking Guardian Caps Rejected by NFL for Regulation Play, OK'd for Practice - Core77
NFL training camps: Guardian Cap players are wearing over helmets, why