Wednesday 6 December 2017 10:06, UK
Since 1999, the premier clubs from around Europe have met back-to-back within their European Cup Pools around this time of year.
The unique format for the club game has produced some immense match-ups and drama throughout the years.
Here are five of the best ones ahead of another weekend of compelling Champions Cup rugby...
In the tightest of 2015/16 European pools, the Ospreys and Bordeaux Begles met first in Swansea, before the return match at the Stade Chaban-Delmas in Pool 2.
Both matches produced extremely close encounters in one of the most exciting back-to-backs in history.
At the Liberty Stadium, the Ospreys came from behind to claim a pivotal 19-16 victory, with Dan Biggar the star of the show, scoring each of the home side's points via a try, conversion and four penalties in an unblemished kicking performance.
Seven days later and the sides shared eight tries in a far more open affair, as Bordeaux turned the tables and clinched a 33-27 victory in a true classic.
In a measure of how unusual this particular pool was, all four sides - Ospreys, Bordeaux, Exeter Chiefs, Clermont - went into the final week with a realistic chance of qualifying.
Exeter proved the only side to advance, finishing on 16 points along with the Ospreys and Bordeaux, with Clermont bottom of the pool with 15 points!
In a sense, the Ospreys and Bordeaux knocked each other out after a staggering two weeks back in December.
One year on from the extraordinary 'Miracle Match' between Munster and Gloucester at Thomond Park, the pair were drawn together in the pool stages once again for the 2003/04 season, meeting in Pool 5.
Placed alongside tournament minnows Benetton Treviso and Bourgoin, the pool was perfectly set up to be decided between the two heavyweights during the January back-to-backs, beginning in Kingsholm.
The previous season, Gloucester, flying high at the top of the Premiership, annihilated Munster at Kingsholm 35-16, requiring Munster to beat them by four clear tries and 27 points in Limerick on the final weekend to progress.
Munster scored four tries and won by...exactly 27 points, thereby eliminating Gloucester from Europe completely. Revenge was in the air.
In an intensely physical clash, Gloucester were on the front foot and denied Munster a losing bonus-point via a Henry Paul penalty with two minutes remaining in a 22-11 victory.
As with the year before, however, this was only part one of the story. In front of a vociferous Thomond Park a week later, Munster plundered 35 points past Gloucester in a remarkable turnaround in fortunes.
The bonus point secured by the Irish province that night, and emphatic margin of victory, ensured Munster topped the pool come the end, finishing above Gloucester only due to their head-to-head record and securing a home quarter-final.
Gloucester were forced to travel to eventual champions Wasps in the knockout stages and lost 34-3. As Munster hooker Frankie Sheahan remarked after their back-to-backs: "A week is a long time in rugby."
During the 2013/14 pool stages, Leinster travelled to Franklin's Gardens to face Northampton in the first half of their Pool 1 back-to-backs.
What ensued was one of the most impressive displays of the professional era.
The Saints had won seven in a row heading into the match and were the form team in England, yet Leinster, inspired by Brian O'Driscoll in his final season, were utterly magnificent.
Free-flowing rugby and tries from O'Driscoll, Jamie Heaslip, Eoin Reddan, plus a Luke Fitzgerald hat-trick, saw them put 40 points on their startled hosts.
O'Driscoll put in one of the performances of his career, silencing doubters after his third Test Lions dropping, with two absolute moments of magic.
A week later, Northampton rocked up to the Aviva Stadium in Dublin and pulled off one of the biggest shocks in recent memory.
An early George North try and Jamie Elliot breakaway effort at the death with Leinster piling on relentless pressure saw them record a scarcely believable 18-9 victory.
The Saints' 42-point swing away from home is one of the most incredible, and unexpected, moments we've witnessed.
While Northampton's turnaround at Leinster is perhaps the most remarkable swing we've seen, the largest in terms of sheer numbers occurred a decade earlier.
For that, we have to go back to the 2003/04 season and Leicester Tigers' two meetings with Ulster in Pool 1.
With both sides having lost one and won one in the opening two rounds of Europe, the pair went into their fixtures with it all to play for.
The first game at Ravenhill in Belfast saw the northern province blow their English counterparts away, winning out 33-0. A Tigers side which had won four league titles on the spin between 1999 and 2002, as well as two European Cups in 2001 and 2002 no less.
Just six days later, Ulster traveled to Welford Road and were unceremoniously thrashed 49-7.
In the week he announced his international retirement, Martin Johnson unleashed a maelstrom of intensity and physicality, the like of which few could have lived with that day.
From a 33-0 defeat to 49-7 victory. A swing of some 75 points. Astonishing.
Heading into the 2012/13 season, Leinster were the team to beat.
Having won three of the previous four European Cups in 2009, 2011 and 2012, they had established themselves as the number one club side in the northern hemisphere.
Drawn alongside Clermont, Exeter and Scarlets, the back-to-backs against Clermont were always likely to decide who would top the pool.
Clermont had always proved a tricky assignment for anyone but their 2012/13 vintage was a cut above any side they had produced before.
The first meeting at the Parc des Sports Marcel Michelin was a tense affair, as the boot of Morgan Parra, plus a Brock James drop goal, outscored the boot of Johnny Sexton 15-12.
Coming away with a losing bonus point wouldn't have been viewed as the end of the world for Joe Schmidt's side, but the return leg was played with them under extreme pressure.
In front of nearly 50,000 at the Aviva Stadium, Clermont belied the cliche that French teams can't travel, pulling off a marvellous 28-21 victory, essentially dispelling their esteemed hosts to a pool stage exit.
Having been knocked out by Leinster at the semi-final stage the previous season, the men in blue took on a bête noire type image for Clermont.
Knocking out the defending champions in their own backyard in a feverish atmosphere is well deserving of a place in our top five.
