The sentinel ritual is precise and exacting: March 21 steps down a black mat behind the tomb, face east for exactly 21 seconds, turn and face north for 21 seconds, then take 21 steps down the mat and repeat the process. The tomb guard then “executes a sharp shoulder-arms movement,” placing a rifle on the shoulder closest to visitors, “signifying that he or she stands between the Tomb and any possible threat,” according to the Arlington National Cemetery website.
