The Changing of the Guard in London is at one level very simple. The guards at various places change duties. Yet, at anther level, trying to explain what is going on is like trying to explain the rules of cricket to someone with no knowledge of the sport.
Changing of the Guard - Basic Information
Marching Band at the Changing of the Guard, adapted from geograph.org.uk
For basic information, if you want to watch it then there are three places to choose from; Horseguards Parade, St James's Palace on Marlborough Road or, most popularly, outside Buckingham Palace. The ceremony starts at 11am at Horseguards parade while at the palace things begin around 11:30. It takes place daily during the summer months and is all over by about 12:30. It is watched by over 2 million people each year, is full of pomp, ceremony, marching bands and free.
While the Foot Guards have been guarding the monarch since the 17th century, what is seen today dates back to the 19th century, when Queen Victoria took up residence at Buckingham House in 1837. If you want a more in depth understanding of what is going on then read on.
Note: This could get confusing, if you want the official version see the Royal Website or the video below.
The Changing of the Guard, also known as the Guard Mounting, involves the Old Guard who have been 'in', or on duty, for the previous 24 hours being replaced by the New Guard who have been 'out'. The Old Guards congregate waiting for the New Guards to arrive.
At Horseguards, the mounted guard, currently known as the Queen's Life Guards, is either a Long Guard or Short Guard depending on whether the monarch is in or not. If the monarch is out then the guard coming in is Short, if the monarch is in then the New Guard is Long. In either case, the old guards going out will not have changed their Long or Short status from when they came in 24 hours earlier. The two guards assemble on Horseguards Parade for a change-over ceremony before the New Guard take up posts here and the Old Guard heads out up The Mall to their barracks in Knightsbridge.
At the other end of The Mall, the Old Guards going out assemble on the fore courts of Buckingham Palace and St James's Palace. They then wait for The New Guards coming in to allow them out. Incoming guards march out from Wellington Barracks to Buckingham Palace accompanied by a brass band. The Old (outgoing) and New Guards (incoming) then face each other and present arms. The captains of the Old and New approach and touch left hands before heading off to the Guards Office. This symbolizes the handing over of the palace keys and accompanying responsibilities from the outs to the ins, or Old to the New. New Guards then take up their posts at the palace.
Throughout the handover, a brass band will play a variety of music, both old and new, with Abba and James Bond theme tunes being among the most popular. It probably helps distract the crowd from trying to understand all the ins and outs of what is going on.
A small detachment of the incoming detach and go out to St James's Palace to allow the outgoing guard therefrom to come into Buckingham Palace. Here, they join the rest of the Old, who en masse head back out for Wellington Barracks leaving the New Guard in place.
This marks the end of the ceremony and the Changing of the Guard, and time to find a nice cup of tea.