Queen Elizabeth II celebrates her 86th birthday today. According to tradition, she will celebrate in private but the occasion will be marked with a 41-gun salute in Hyde Park.
Leading the ceremony is the King's Troop - a mounted, ceremonial unit consisting of 111 trained horses. In groups of six they will pull six Second Wold War-era field guns into place for the gun salute.
The gun salute involves 41 blasts at 10-second intervals. Whilst a Royal Salute normally comprises 21 guns, this increases to 41 rounds if fired from a Royal Residence or Royal Park.
In 2006, Her Majesty celebrated her 80th Birthday with a walkabout in the streets outside Windsor Castle to meet well-wishers.
It is not known whether she will do the same this year, although she will personally open a selection of the cards she receives from people around the world.
Although the Queen's actual birthday is on April 21st, her so-called "official" birthday is celebrated on a different day, usually in late May or early June. The exact date varies from country to country within the Commonwealth.
In the UK, the occasion is marked by the Trooping of the Colour ceremony - a tradition stretching back to 1748.
The ceremony consists chiefly of the Queen proceeding down The Mall to inspect her troops before returning to Buckingham Palace for a march past followed by an RAF fly-by.
See a video of the Trooping of the Colour ceremony from 1986 below:
This year's Trooping the Colour will take place on 16 June 2012.