Sixteen shirts line the walls of the main reception at the Stoop.

All are Harlequins jerseys, but none are in the club's trademark colours of blue, magenta, brown and grey.

One is camouflage khaki, another has neon laser stripes jagging across it, a third glows in the dark.

They haven't featured in finals or trophy lifts. Yet, they have pride of place.

They are all special editions, created for their Big Game, an annual festive fixture for which Harlequins make the half-mile trip from their 14,800-capacity Stoop to the 82,000-seater Allianz Stadium.

And, invariably, they fill it.

Since the first Big Game in 2008, it has become the biggest yearly fixture in club rugby anywhere in the world.

This is the logic and logistics behind a much-admired, often-copied, but unsurpassed concept.

And this is where you find 67,000 extra paying fans.

"I'll never forget running out at the Stade de France, and there being can-can girls, knights jousting on horseback and gymnasts doing these amazing flips on the pitch," said scrum-half Danny Care.

"I was a 21-year-old kid just thinking, what on earth is going on?"

It was 8 December 2008 and Harlequins were part of the cast for Stade Francais' latest gaudy jaunt to France's national stadium.

Media magnate Max Guazzini was Stade's owner and known for his flamboyant marketing of the team. Eye-popping kits, calendars featuring naked players and celebrity visits to the changing room fed the hype.

Staging matches at the Stade de France, and attracting near 80,000 crowds in the process, was another of his ringmaster tricks.

It may have come as a shock to Care, but Harlequins chief executive Mark Evans had been watching for a while.

By the time Quins and Care played in Paris, Evans had already booked out Twickenham for three weeks later and a match against Leicester.

"Stade Francais had been doing it for three years or so at that point," said Evans.

"Paris is not a big rugby city - it was even less so back then - and I thought if they can get 80,000 people in on a combination of staging an event and pricing which made a large percentage of seats affordable, I was sure London could do the same."

Not everyone agreed.

Festive sporting crowds weren't a London phenomenon because people leave the city for Christmas. Some thought Twickenham - eight times the size of the Stoop at the time - was too big a stage.

Those were the theories. Evans' faith was repaid with a different reality.

"Lots of people said it wouldn't happen or it would be a disaster, and we were nervous - until it was four weeks out and we had sold 30,000 tickets," Evans said.

The previous record attendance for a regular-season Premiership fixture was 23,709. By the time kick-off came around, Quins, decked out in pink and blue, had shifted 50,000 tickets – a sellout after the authorities limited attendance because of transport issues.

The scoreline from that match was level - a 26-26 draw - and so was the balance sheet.

With tickets priced from £10, a 50,000-strong crowd was the break-even point. But, more importantly, a tradition had been established.

A firework-filled photo of that first Big Game filled the front page of the Sunday Times sport supplement the following day.

Twelve months later, the match ball arrived in the talons of an eagle. On another occasion, it was delivered to the centre of the pitch by a hovercraft.

X Factor contenders were booked to perform on the pitch, while Evans splashed out on a wraparound advert that plugged the game on the front of the London Evening Standard.

The second and third editions of Big Game attracted around 75,000 fans. The fourth pulled in 82,000.

"The idea was to bring people together but, let's not mess about, the idea was also to make money," said Evans.

"We were trying to create an event that in the long run would make a significant difference to the business.

"I was stood in a supermarket queue in Roehampton one December and two women behind me were having a conversation about what they were doing over Christmas.

"And one said to the other 'well, we always go to the Big Game'.

"I knew then that we had them. It was done. From then it was a case of 'don't be stupid, don't screw it up, don't be too greedy'."

Laurie Dalrymple sits in the seat Evans once occupied.

From his chief executive's office, he can see a board tracking Big Game ticket sales week by week.

Last Friday, a final sticker was added, marking Big Game 16 - Saturday's match against Leicester - as another 82,000 sellout.

"It is critical for us as a business," Dalrymple said.

"This year's will be the highest-grossing Big Game the club has ever had. The turnover represents probably 8-10% of the club's turnover [approximately £27m annually in the past two years of full accounts]."

As profits have risen, so have prices, and Big Game's break-even point has dropped.

The cheapest tickets, pitched at £10 for Big Game 1, are now £36 for adults.

Once 40,000 have been sold, the fixture is more profitable than had it been played at the Stoop.

The annual task of finding the fans to fill those seats falls to chief marketing officer Adrian Wells.

In previous years, he oversaw a ground offensive of billboards, bus-stop adverts and leaflet drops in the areas linked to Twickenham by the train line. Clapham was a particularly profitable hunting ground.

Now, the sell mostly happens online.

"Our marketing is much more digitally led, much less spatially targeted," said Wells.

"It is around a type of person. We can build 'lookalikes' of people who have bought tickets before: rugby fans, big event fans, people who have engaged with Harlequins, have engaged with the music act, or have bought tickets to other things around London.

"And then we do similar to attract away fans which can be 15-17% of the audience depending on who we play."

As the tickets sell, the data stacks up.

The Big Game crowd is younger and more female than the Stoop regulars, creating new markets and potential season-ticket holders of the future.

Harlequins have grown the largest social media reach in the Premiership and are in the top 10 of clubs worldwide, with Big Game providing a host of shareable moments.

Not all of the fixture's benefits are as easy to quantify though.

This year's match has a sub-title - The World is Our Stage - with the special edition kit and pre-game laser show riffing on vintage maps and giving long-time sponsors DHL a share of the spotlight.

The spectacle is regularly picked as one of ITV's live games, bringing in a free-to-air audience.

And it is a cherished part of the calendar for the players.

Each year, they walk from the Stoop, across a bridge crossing the Chertsey Road, through a gauntlet of home fans and into Twickenham.

South African prop Babalwa Latsha, who grew up in the Khayelitsha slum township in Cape Town, turned the corner to find her face up in lights on the side of Allianz Stadium last December and promptly dissolved into tears., external

"She is a massive role model of us and her whole country too and seeing what that moment meant to her and girls that look like her, was awesome," said team-mate Lagi Tuima, who will be part of Harlequins' attempt to break the record for a club women's game attendance once again this year.

Care has played in all 15 Big Games so far and has booked two hospitality boxes for his family to attend this year as he appears at the stadium for potentially the final time.

"To see it packed out for a club game, to go there with your mates, the lads you spend more time with than your own family and play at that stadium together - you can never underestimate how cool that is," Care said.

Keeping it cool is a constant challenge.

Ice sculptures have featured in the past. A free fun fair is a constant. Superstar DJ Pete Tong played a bespoke remix of club anthem The Mighty Quinn, while Faithless' Sister Bliss wore a sparkling diamante Quins shirt.

A staff Whatsapp group buzzes year round with ideas about what might add to the occasion, with inspiration taken from everywhere between a drone display at a college football game in Texas, external and a water-effect feature at Kew Gardens' Christmas lights.

A host of teams have tried similar.

Saracens, who have previously booked Wembley, will host Harlequins in March at the 62,850-seater Tottenham Hotspur Stadium for The Showdown. In May, Bristol will host West Country rivals Bath at Cardiff's 74,500-capacity Principality Stadium for their first Big Day Out.

Glasgow played Edinburgh in front of more than 27,000 fans at Hampden Park last weekend, while Leinster sold 80,000 tickets when they took Munster to Croke Park in October.

Quins themselves launched a second annual fixture at Twickenham in 2022 - Big Summer Kick-Off - and Dalrymple aspires to stage an exhibition match overseas, with Singapore a dream destination.

"It is good for rugby," said Wells of the trend. "Our sport needs that sort of investment and show around its highest-profile moments.

"We are happy to share and help grow the game. Equally, we want to make sure ours is the best."

The rest of rugby is still Big Game hunting when it comes to matching Quins' money-spinning spectacular.