The Queen with Canadian Governor General David Johnston at Canada House in London. Reuters / Toby Melville There are many unwritten rules that come along with hanging out with a royal.

While the royal website says there are "no obligatory codes of behaviour when meeting the Queen or a member of the Royal Family," it adds that "many people wish to observe the traditional forms."

One of these unwritten rules is that members of public do not touch Her Majesty the Queen.

Canada's governor general, however, went against acceptable royal etiquette on Wednesday by touching Her Majesty's elbow to "ensure she didn't slip" during her visit to Canada House with the Duke of Edinburgh.

Queen Elizabeth II, 91, was visiting Canada House in Trafalgar Square to mark the country's 150th birthday.

When ascending the steps, Canadian Governor General David Johnston lightly touched her elbow to ensure she didn't stumble on a "slippy" carpet. He did it again when she left the building.

"I'm certainly conscious of the protocol," Johnston told CBC News.

"I was just anxious to be sure there was no stumbling on the steps. It's a little bit awkward, that descent from Canada House to Trafalgar Square, and there was carpet that was a little slippy, and so I thought perhaps it was appropriate to breach protocol just to be sure that there was no stumble."

According to the royal website, men should bow their necks when they meet the Queen or a member of the royal family, while women should curtsy.

The correct way to initially address the Queen is "Your Majesty," subsequently followed up with "Ma'am."

Other members of the royal family should be called "Your Royal Highness," also followed by "Sir" and "Ma'am."