Time was when a meeting of two English teams in European football's most important club competition was a rarity, dependent on one of them being the holders. In recent seasons, with three or four Premier League sides involved in the knockout stages, it has become commonplace, without losing the capacity to produce occasions of immense emotional intensity.
Both Chelsea and Manchester United have sampled those nights – above all in the epic Moscow final of 2008 – which does not dilute the anticipation ahead of the two-leg quarter-final over the next 10 days. What neutrals have to hope is that as well as providing entertainment as gripping as Chelsea's duels with Liverpool or United's against Arsenal, the outcome is not decided by a refereeing controversy.
That has been the case in the clubs' most recent encounters, as Sir Alex Ferguson has been keen to point out; indeed, his vehemence in doing so with reference to Martin Atkinson, who was in charge when United lost 2-1 to a late penalty at Stamford Bridge, brought a touchline ban from domestic games.
Before that, there was Didier Drogba's goal a year ago today, shown to be offside, which decided the key match at Old Trafford and thus the League title; and earlier the disputed free-kick from which John Terry won the League game in London.
Many will have been amused to hear Ferguson of all people claiming that United must not get "paranoid" about all this but at United's training ground on Friday he declined the opportunity to talk in detail about refereeing, confining himself to saying: "They've had some important breaks against us. In the last four games, major decisions have gone their way. But we beat them in the European final. We were the better team in the European final and we were the better team a few weeks ago at Chelsea. We could do with a bit of luck against them."
If luck and contentious decisions do not decide the tie, what might? Could it be, to isolate a single factor, how Fernando Torres fares against a recently depleted United defence? In Liverpool colours he was regularly a thorn in their side, one of the few strikers in the country uncowed by Nemanja Vidic's physicality and able to master him for pace. In Chelsea blue he has yet to convince, and worse, has forced a change of formation not necessarily to the team's benefit.
Ferguson, wary of offering a hostage to fortune, was not prepared either to talk him down or build him up, other than suggesting that choosing to buy him in mid-season underlined the importance to Chelsea's owner of winning a first Champions' League. "I think [Roman] Abramovich has earmarked this as the thing he wants to win. That's the obvious reason to bring in a player at £50m in January. Players sometimes need time to settle, they're all different. It was a massive signing, a lot of money, but it's difficult to assess."
What United's manager is concentrating on is who he will have in opposition to Torres in London on Wednesday. Not Rio Ferdinand, who has at least resumed training, which leaves Jonny Evans or the seemingly more reliable Chris Smalling alongside Vidic. "It's important for us to get a strong side out defensively, that'll give us a real good chance," Ferguson said, adding a vote of confidence in Smalling: "The games he played, he was very exciting. Ever since Rio went out, he's [been] in a great moment, his experience and confidence have got better. Defenders are normally 24 or 25, he's three years off that but the boy's got a great chance."
Whether or not Carlo Ancelotti wanted Torres brought in halfway through the season, he is now charged with fitting him into the side, and the result may determine the manager's future. What it has tended to mean so far is either an unbalanced attack, with both Torres and Drogba wanting the central space, or a change of formation to a 4-4-2 in which there is no natural width down the right and Nicolas Anelka misses out.
The speculation is that Drogba will depart in the summer, perhaps to rejoin Jose Mourinho at Real Madrid. In the meantime, Ancelotti insists the two strikers can play together and that his Ivorian must live with the new arrangements: "I spoke with Didier about this. He was told that Torres is here to improve the power of the team. For this reason he is happy. But Didier is intelligent and he understands very well. I think they can play together because they are not the sort of player who likes to stay at the front of the pitch and not move. A lot of times Didier goes outside to put crosses in. Torres is the same."
So he must hope, though the evidence thus far is less than compelling. Back in the dug-out, however, Ferguson knows that he will be sitting – and frequently standing – opposite a formidable adversary, whose varied Champions' League experiences in charge of Milan (winning two finals and losing one) have given him a special feeling for this competition.
"It is my favourite trophy," says Ancelotti. It has defined my career as a player and manager, with the biggest highs and the biggest lows. At Milan there is a lot of difference between Serie A and the Champions' League. They were always focused on the Champions' League because for the image of the club, for them they think first about the Champions' League. Here it is a little bit different. But it is also important for us to win this title. It's not an obsession, but it is a dream."
The dream should survive 90 minutes at Stamford Bridge. The crunch will come a week on Tuesday, when United's ability to have taken an away goal back to Old Trafford with them could be as critical a factor as any, including Torres's unpredictable form.
Chelsea v Manchester United is on ITV1 on Wednesday, kick-off 7.45pm
Classic English European Cup clashes
European Cup first round 1978-79: Nottingham Forest beat Liverpool 2-0 on aggregate
Brian Clough's Forest had won the league by seven points the previous season but went in as huge underdogs against Bob Paisley's European champions. In the first leg at the City Ground, Garry Birtles tapped in before defender Colin Barrett doubled the lead. A Liverpool side containing Kenny Dalglish and Alan Hansen couldn't find a way past Peter Shilton at Anfield. Forest went on to win consecutive European Cups.
Champions' League semi-final 2004-05: Liverpool beat Chelsea 1-0 on aggregate
The infamous "ghost goal". After a stalemate at Stamford Bridge, it was to be one of the most memorable European nights at Anfield. Liverpool went ahead after just three minutes when Luis Garcia (pictured right) poked the ball in. William Gallas desperately tried to clear but the referee gave the goal – which Jose Mourinho, Chelsea's manager at the time, claims to this day should not have stood.
Champions' League quarter-final 2007-08: Liverpool beat Arsenal 5-3 on aggregate
After a dull 1-1 draw at The Emirates, a belter at Anfield. Abou Diaby gave Arsenal the lead but Sami Hyypia and Fernando Torres put Liverpool in command. An inspired run from Theo Walcott, who travelled the length of the field to set up Emmanuel Adebayor for a tap-in, gave the Gunners hope but Kolo Touré then fouled Ryan Babel in the penalty box and Steven Gerrard made no mistake from the spot. A last-minute Babel goal wrapped things up and Rafa Benitez faced another semi-final against Chelsea.
Champions' League final 2008: Manchester United drew 1-1 with Chelsea after extra-time (United won 6-5 on penalties)
The first all-English European Cup final. Cristiano Ronaldo gave United the lead but Frank Lampard equalised. In the shoot-out, Chelsea captain John Terry missed a chance to win it before Edwin van der Sar saved from Nicolas Anelka to give United a third European crown.
Champions' League semi-final 2008-09: Manchester United beat Arsenal 4-1 on aggregate
United took a one-goal lead to the Emirates, where Arsenal had not lost in Europe, but that record was in tatters after 11 minutes. Cristiano Ronaldo played in Ji-Sung Park to score, before a piece of brilliance from Ronaldo caught Manuel Almunia off guard from a 35-yard free-kick. Ronaldo added a third after the break before Robin van Persie netted a late consolation.