This is another one that you should know by default. When the reigning monarch walks past, gentlemen must bow and ladies must curtsy. That doesn't just apply to us common folk either; even other members of the Royal Family observe this protocol, as seen in this picture, where William & Kate are showing deference.
This tradition isn't as demanding as it used to be. You're fine to bend one knee and bow from the head these days, but during the time of the Tudors, anyone in the presence of the reigning King or Queen was expected to bow to the waist and stay there until acknowledged and told to desist. They must have had incredibly powerful calf muscles. It was even worse if you were leaving the room, in which case the correct procedure was to bow three times while walking backward. We wonder how many people banged into the door?