In American football, why don't teams sub quarterbacks in and out
History of The Football Helmet - Atavus
Circa 1898 Chicago Style Head Harness Leather Football Helmet
Delaware State University Archives and Special Collections
Delaware State University Archives and Special Collections
Researchers test old-style leather helmets vs. modern ones and
Design and Virtual Testing of American Football Helmets–A Review
Football helmet - Wikipedia
The Football Helmet timeline