LONDON (Reuters) - Touching Queen Elizabeth has long been considered one of the greatest taboos surrounding the British monarch, one of the unwritten rules about how people are expected to behave around the royal family.

Queen Elizabeth II smiles after she started the London Marathon, from Winsdor Castle, which was relayed to big screens at Blackheath, setting off 40,000 runners on the 26.2 miles to The Mall, in Windsor, Britain April 22, 2018. Chris Jackson/Pool via Reuters

When American actress Meghan Markle walks down the aisle of St George’s Chapel at Windsor Castle to marry the queen’s grandson Prince Harry on May 19, she will have to have mastered those protocols.

Lucy Hume, the associate director at Debrett’s, a professional coaching company founded in 1769 and an authority on modern British etiquette, explains how members of the public should act if they happen to meet a member of Britain’s royal family.

WHAT SHOULD I DO IF I MEET A MEMBER OF THE ROYAL FAMILY?

“One of the key things to bear in mind is how to greet a member of the royal family when you meet them for the first time, and it’s customary in a formal situation for women to curtsy - a brief bob is sufficient - and for men to bow from the neck.”

WHAT SHOULD I DO IF I MEET A ROYAL AT A LESS FORMAL EVENT?

“Sometimes you might encounter a member of the royal family at a reception or a less formal occasion, in which case it might be appropriate to shake their hand, but it’s best to wait for them to offer their hand first before you reach out yours.”

WHAT DO I CALL THE ROYALS WHEN I MEET THEM?

“If you are introduced to the queen, you address her as ‘Your Majesty’. Any other members of the royal family are addressed as ‘Your Royal Highness’ and for women you thereafter would call them ‘Ma’am’ and, for men, you would call them ‘Sir’.”

CAN I ASK A ROYAL FOR A SELFIE?

“It’s not usually appropriate to do so. So if you do want to take a photograph, ask their permission first, possibly get somebody else to take a photograph but it’s important to respect that person’s personal space.”

IS IT OKAY FOR ME TO PUT MY ARM AROUND A ROYAL?

“Best not to initiate personal physical contact with a member of the royal family. Again, it may be that they offered to give you a hug or to put their arm around you, but usually wait and see what’s expected or what’s appropriate for the event.”

WHAT HAPPENS IF I GET SOMETHING WRONG?

“You’re not going to be in trouble. There are no official legal rules in place. Apologize if you feel you may have caused offense, but try not to panic, and stay calm.”

WILL THE ROYALS BE KIND TO ME IF I MESS UP?

“It would seem from what we’ve seen of members of the royal family in public that they are very accustomed to a variety of different situations. They meet people from different backgrounds all the time and they’re very accustomed to putting people at ease; it’s unlikely you would cause offense.”