Before every sitting of the House, the Speaker's procession leaves the Speaker's House inside the Palace of Westminster and heads for the Commons chamber.
The Serjeant at Arms, carrying the Mace, and a doorkeeper walk ahead of the Speaker. Some members of the Speaker's staff - his chaplain, secretary and a trainbearer - follow behind.
As the procession reaches the central lobby of the Houses of Parliament, one of the policemen on duty shouts: "Hats Off Strangers!"
Then, policemen in helmets and any members of the public wearing hats are required to remove their headgear as the procession passes.