Arlington National Cemetery and the changing of the guard ceremony (historical)

The Changing of the guard
The military guard at the Tomb of the Unknown Soldier is changed in an elaborate ceremony which happens every hour on the hour from October 1 through March 31, and every half hour from April 1 through September 30.

Twenty-four hours a day, soldiers from the 3rd U.S. Infantry Regiment, known as "The Old Guard," stand watch over the Tomb. The Tomb Guards, also called Sentinels, are chosen for this prestigious and highly selective post only after rigorous training and a demanding series of examinations (see below). The Old Guard has held this distinguished duty since 1948.  

An impeccably uniformed relief commander appears on the plaza to announce the changing of the guard. Soon, the new Sentinel leaves the Tomb Guard quarters and unlocks the bolt of his or her M-14 rifle, signaling to the relief commander to begin the ceremony. The relief commander walks out to the Tomb and salutes, then faces the spectators and asks them to stand and remain silent during the ceremony.

The relief commander conducts a detailed white-glove inspection of the weapon, checking each part of the rifle once. Then, the relief commander and the relieving Sentinel meet the retiring Sentinel at the center of the black mat in front of the Tomb. All three salute the Unknown Soldiers who have symbolically been given the Medal of Honor. The relief commander orders the relieved Sentinel, "Pass on your orders." The current Sentinel commands, "Post and orders, remain as directed." The newly posted Sentinel replies, "Orders acknowledged," and steps into position on the mat. When the relief commander passes, the new Sentinel begins walking at a cadence of 90 steps per minute.

The Tomb Guard marches exactly 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.  (The number 21 symbolizes the highest military honor that can be bestowed, the 21-gun salute.) Next, the Sentinel executes a sharp "shoulder-arms" movement to place the weapon on the shoulder closest to the visitors, signifying that he or she stands between the Tomb and any possible threat.

When not "walking," the Tomb Guards spend their duty time in quarters below the Memorial Display Room of the Memorial Amphitheater, where they study cemetery history, clean their weapons and help the rest of their relief prepare for the changing of the guard.