The Tomb of the Unknowns is guarded 24 hours a day, 365 days a year, in any weather, by members of the elite 3rd U.S. Infantry with headquarters in Fort Myer, Virginia. The guard is changed every hour on the hour from October through March, and every half hour between April and September. The tomb guard marches 21 steps down the black mat behind the Tomb, turns, faces east for 21 seconds, turns and faces north for 21 seconds, then takes 21 steps down the mat and repeats the process. After the turn, the sentinel executes a sharp "shoulder-arms" movement to place the weapon on the shoulder closest to the visitors to signify that the sentinel stands between the Tomb and any possible threat.
My video:
http://www.youtube.com/watch?v=UOlJ_WFtfPA
