<P> Twenty - one was chosen because it symbolizes the highest military honor that can be bestowed--the 21 - gun salute . </P> <P> The mat is usually replaced twice per year: before Memorial Day and before Veterans Day . This is required because of the wear on the rubber mat by the special shoes worn by Tomb Guards . The sentinels have metal plates built into the soles and inner parts of their shoes to allow for a more rugged sole and to give the signature click of the heel during maneuvers . The sentinels wear sunglasses because of the bright reflection from the marble surrounding the Tomb and the Memorial Amphitheater . </P> <P> On the ground not covered by the mat, a rust pattern in the tile can be seen that corresponds to the precise steps taken during the changing of the guard . The metal from the guards' boots causes the brown rust markings on the stone . On the mat itself, footprints caused by standing guard are also visible . </P> <P> While Arlington National Cemetery is open, during the day in summer months from April 1 to September 30, the guard is changed every half hour . During the winter months, from October 1 to March 31, the guard is changed every hour . After the cemetery closes to the public (7 p.m. to 8 a.m. April through September, and 5 p.m. to 8 a.m. October through March), the guard is changed every 2 hours . The ceremony can be witnessed by the public whenever Arlington National Cemetery is open . </P>

How many in the tomb of the unknown soldier