Answer: Scrunchies. We are owners of the hair relic, whose popularity peaked in the early 1990s and was then shunned to the back of bathroom cabinets.
Harry's other half Cressida Bonas wore hers to the theatre and at last summer's festivals, presumably to distance herself from her posh pals in the Made in Chelsea cast.
Even young actress Selena Gomez has shown off a scrunchie ponytail on a red carpet outing.
And it's not just the fabric-covered hair bobble that has crept back from that decade and into 2014.
Trainers - the flavour of the month is Adidas Gazelles - are now being worn with pride outside the gym and high waisted faded jeans, dubbed 'mom jeans', are selling out.
Plaid, which Alicia Silverstone made desirable as Cher in Clueless, is on every shop rail and Kate Moss-type slip dresses are back, too.
The 1990s have been bubbling up for a while but the return of the scrunchie signals new proportions.
Go out into Buchanan Street and scan the crowds. You'll notice all of the clothing mentioned, and possibly a bum bag.
Maybe even the humble fleece will be a trendy item just in time for the weekend's storm.
London always seems to be ahead of the trends. It's probably because their Oxford Street Topshop is the size of a small town.
My Dalston-based pal proved this when she turned up at Christmas looking like she walked straight out of an episode of My So Called Life.
Her velvet maroon- coloured scrunchie looked so good I bought a pack.
Despite the call of the 90s with its Bungle from Rainbow, Brit pop and all-night raves memories, one decade is still better.
According to a new survey by style website Female First, the 1960s was voted by the public as the most fashionable time.
IT GAVE us the mini skirt, shift dresses, monochrome and the most unobtainable figure of all time - Twiggy's, of course.
We love nostalgia, especially when it comes to fashion. Other decades that keep coming back are the 20s, thanks to its flapper dresses, and the 1950s with its nipped in waists.
The 1970s are enjoying a moment thanks to American Hustle and its flamboyantly-dressed characters but we are not ready for bell bottoms.
Trends are recycled over and over again. Do we ever invent anything new?
Here's a depressing thought: The only fashion phenomenon I have registered in the past few years is the onesie.
The only period we try to avoid is the 80s. Nostalgia dressing only works if the decade was fashionable in the first place.
Thatcher power suits and Dynasty shoulder pads were never a good look.