The Duke and Duchess of Cambridge seldom indulge in public displays of affection as they are representatives of the Commonwealth.
On March 17, 2018, Prince William and Kate Middleton celebrated the St. Patricks Day by attending the annual event with the Irish Guards at their barracks in West London.
As reported by House Beautiful, Middleton showed off her 8-month baby bump, whereas William gently placed his hand on her lower back.
While they're not prohibited from showing displays of affection publically, it's frowned upon at official events. However, the same rules do not apply to casual occasions, for instance, any sports events.
As a symbol of respect, the Royal couple is not allowed to not exaggerate it. However, a mild pat on the back, shoulder or elbow is still allowed.
When a couple from the royal family touch one another publically, it instantly grabs the media's attention.
The rules are slightly lenient for Prince Harry and his fiance, former Suits actress Meghan Markle. They often show displays of affection once they are in public, even holding hands.
During an interview with BBC in November 2017, Markle was seen holding the Prince's hand. She also placed her hand on his knee as she was explaining how they met.
Prince Harry and Markle's behavior is considered to be 'atypical for royal engagements.' Standards for them slightly differ from those expected from for his brother and sister-in-law.
Prince William and Middleton are considered to be the flagbearers of the monarchy and are expected to be the future King and Queen.
They are expected to 'appear slightly prim and proper' anytime they are stepping out in public, especially when they are attending an official event.
The Duke and Duchess are expecting their third child, whose arrival would possibly fall on the same date as the couple's seventh wedding anniversary, April 29. After the child's birth, Prince Harry will then be sixth in line to the throne.