The story is told in Harry's memoir, called Spare, and he says the term was often used to describe him, within his own family.

"They would say it without a spirit of judgement, but straight out. I was the shadow, the supporting actor, the plan B," he writes, in a translation of the book's Spanish edition.

"I was brought into this world in case something happened to Willy," he writes, using the nicknames that saw Prince William as "Willy" and Prince Harry as "Harold".

The saying "an heir and a spare" refers to aristocratic families needing an heir to inherit a title or an estate, and the "spare" as the younger sibling who could be the replacement if anything happened to the heir before he or she has their own children.

It clearly annoyed Prince Harry enough to use it as a title for his book, and it taps into the longstanding difficulty of this uncertain royal understudy role, where there's wealth and privilege but no obvious sense of purpose.

"It's a non-position," says royal expert Professor Pauline Maclaran, from the Centre for the Study of Modern Monarchy, Royal Holloway, University of London.

"There's no clear role apart from shaking hands and being pleasant to people," says Prof Maclaran.

A life of pointless luxury might have its decadent charms, but it also carries a heavy risk of unfulfillment and lack of direction.

So much so that Prof Maclaran says that a modern, slimmed-down monarchy should either find better defined roles for such individuals, or else release them from any royal expectations, once they've slipped down the pecking order of succession.

Royal historian Ed Owens says Sweden and Denmark are examples of where such an approach has been taken, "downsizing" the royal families, so that individuals who might have been marginal "spares" can have their own private lives "unfettered by royal responsibilities".

Mr Owens says that Princess Margaret, younger sister to the late Queen Elizabeth II, is an example of the pressures put upon such siblings, in a way that remains relevant to the problems raised by Prince Harry.

He says Princess Margaret faced having her private life "taken apart" by the media - something that he says would be much less likely to happen to the monarch or those directly in line of succession.

For the second child, with no role expected beyond dutifully "playing second fiddle", it means facing tougher and more "mischievous headlines", with less expectation of protection from scrutiny, says the historian.

"More marginal royals are seen as fair game. The dignity accorded those in direct line is not afforded to those younger royals," he says.

This echoes Prince Harry's description of the spare as a "back-up, distraction, diversion".

Prince Andrew is another whose attempts at finding a role have not exactly ended well. Even before the scandal involving sex assault claims, which he denied, he had gained the nickname of Air Miles Andy for his many overseas trips.

But there have been positive outcomes. George VI, a shy and initially reluctant monarch, had been Edward VIII's younger sibling but stepped up to the plate as King after his childless brother abdicated, and proved to be a leader in wartime.

His father, George V, had been another second son who came to the throne. His older brother had died at the age of 28 in an outbreak of flu in 1892.

Apart from the hereditary principle that puts the "spare" into second place, Prof Maclaran suggests looking at it through the prism of family dynamics.

She says they can have "second-child syndrome", rebellious and resentful of the older child who has been given more status and responsibility.

This can create its own sibling tension, Prof Maclaran argues, with the older child also getting irritated by the lack of responsibility shown by the younger. It's a psychological tinder box as well as a constitutional fixture.

There are also historical templates for the younger royal being cast in a negative light, accused of dissolute and disreputable behaviour or as a rival threatening to undermine the authority of the elder.

Think of Bad King John versus Good King Richard, in the 12th Century power struggle between brothers, that was depicted in a 1970s Disney movie.

But in the 21st Century there's no need for any restrictions or negativity towards "spare" royals, says author and historian Sir Anthony Seldon.

He says that Prince Harry and Meghan have great potential to do good and could have a "massive future, rather than be spent forces," for the monarchy if "wisdom and generosity prevails".

"Someone has to be the grown-up here. You have two very understandably damaged brothers and this will drag on and on and start causing serious damage.

"It is still recoverable, but there has to be an accommodation between the brothers.

"No-one is winning from the current war," warns Sir Anthony.