The Queen's Life Guard is the mounted guard at the entrance to Horse Guards. Horse Guards is the official main entrance to both St James's Palace and Buckingham Palace (a tradition that stems from the time when The Mall was closed at both ends); however, sentries have been posted there since the Restoration, when the Palace of Whitehall was the main royal residence. The guard is on horseback from 10am until 4pm, with the two sentries changing every hour. From 4pm until 8pm a pair of dismounted sentries remain. At 8pm, the gates of Horse Guards are locked, and a single sentry remains until 7am.
When The Queen is in London, the Guard consists of 1 Officer, 1 Corporal Major (who carries the Standard), 2 Non-Commissioned Officers, 1 Trumpeter and 10 Troopers. This is known as a Long Guard. When Her Majesty is not resident in London, the Guard is reduced to 2 Non-Commissioned Officers and 10 Troopers. This is known as a Short Guard.
The guard is usually provided by the Household Cavalry Mounted Regiment, with the Life Guards and the Blues and Royals alternating. When the HCMR leaves London for a month of summer training (and vacation for the horses), the guard is provided by the King's Troop, Royal Horse Artillery. Only two other regiments have had the honour of mounting the Queen's Life Guard; the Royal Canadian Mounted Police did so leading up to the Coronation of King George VI. Lord Strathcona's Horse did so in 2000 during the same period as the Royal Canadian Regiment provided the Queen's Guard.