It's Not a Rule, but Royals Usually Don't Hold Hands For This 1 Important Reason
Along with shutting their own car doors, holding hands is one of those activities that royals purportedly should not engage in, but that is more of an unspoken expectation than an actual royal protocol. PDA in general is considered uncouth among the royals, and even though it's clear that there is affection between the Duke and Duchess of Cambridge, you'll rarely see the two cuddling up in public. There are exceptions, of course, such as at Princess Eugenie and Jack Brooksbank's wedding, but largely William and Kate leave the PDA to Prince Harry and Meghan Markle, who are never shy about showing off their love.
Like most standards of behaviour, royals generally follow the example of the Queen. As Queen Elizabeth II rarely held Prince Philip's hand on public engagements, this has become the unwritten precedent for other royals as well. But while Harry and Meghan's loud-and-proud love for each other is a rare display for royal appearances, it certainly doesn't go against any official rules, and is even considered refreshingly modern by most. According to royal etiquette expert Myka Meier (who spoke to People), "There is no protocol that says [Harry and Meghan] cannot show affection on official engagements, and this gesture makes them relatable and lovable to the public."
Instead of a set rule, the affection shared between royal couples during official outings really just depends on the nature of the event and the preference of the couple. So Harry and Meghan, it's totally fine if you keep showering each other with love (especially given Meghan's exciting baby news!). Check out the gallery to see some of our favourite hand-holding moments among royal couples.