Question: what is the significance of the number 21 at the tomb of the unknown soldier?
Answer: 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.)