The theory that explains why Meghan Markle always walks one step behind Prince Harry.

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Ever since The Duke and Duchess of Sussex Prince Harry and Meghan Markle stepped off their private jet in Sydney almost two weeks ago, they’ve been ridiculously loved up.

Maybe it’s the island breezes, or the Aussie accents, or the increased sun exposure – or the fact they’re having a baby – but the royal couple can’t keep their bloody hands off of each other.

Because Prince Harry and Meghan are holding hands wherever they go, the internet has noticed Meghan always appears to be a step behind her husband.

Not because she’s a bit tired from carrying around the future royal baby in her stomach or is slowing down to take in the scenery.

No, according to an etiquette expert, it’s royal protocol.

“It is standard practice for British royalty to allow the ‘blood royal’ to walk or stand a few paces in front of the royalty who has married in to the family, as with HM The Queen and HRH The Duke of Edinburgh and TRH The Duke and Duchess of Sussex,” etiquette expert William Hanson told Daily Mail.

"Even though TRH the Duke and Duchess of Sussex are currently presenting their own, informal, tactile interpretation of British monarchy, they are still adhering to the precedent with Meghan knowing she, as a non-blood royal, should defer to born royalty."

All the talk of 'royal blood' sounds a bit Harry Potter/cultish, but there is some precedent to support this royal protocol theory. I say some because there's also images that suggest the contrary.

Take Queen Elizabeth II and The Duke of Edinburgh Prince Philip, for example.

For the last 70 years, Prince Philip has always walked behind the Queen at official engagements. Even more obviously so, usually a couple of steps behind, arms crossed behind his back.

His wife is the Queen though, so it makes more sense that the protocol is followed more closely in their relationship.

With Prince Charles and Princess Diana, and The Duke and Duchess of Cambridge Prince William and Kate Middleton, many photos show Princess Diana and Kate walking ahead of their husbands. Oh, and behind. And next to. The evidence is inconclusive.

Neither of those couples held or hold hands as much as Prince Harry and Meghan do either, which brings me to this...

Prince Harry is taller than Meghan. General body composition would suggest his legs are therefore also longer, meaning his strides are longer, meaning he'll always be just that little bit further in front of his wife when they're walking together.

One could argue if it were the other way around, Meghan would be half a step in front of her husband.

Guys, I think we just solved this thing. Case closed.

Catch up on all the highlights from Prince Harry and Meghan's 2018 royal tour below. Warning: PDA ahead.