"Manchester United against Liverpool is always the first game I look out for when the fixtures come out. Maybe the games against City and Chelsea are now just as important because they are more of a threat in the league, but United and Liverpool are the two biggest clubs in English football with huge, worldwide fan bases. It's a massive game."
- Paul Scholes, former United midfielder
Like all the greatest rivalries, it is the common ground that divides the most. Manchester United and Liverpool both hail from largely working-class, immigrant cities with huge Irish populations. Just 35 miles apart in England's North West, both were economic powerhouses who enjoyed a friend/foe relationship by the 19th century.
Liverpool considered itself as the greatest port in the world, gateway to North America for millions and a key trading post for the British Empire. Manchester was "Cottonopolis", the first city of the industrial revolution; hence the phrase "Manchester made and Liverpool trade".
That all began to change in 1878, the year Manchester United were formed as Newton Heath, when a worldwide trade depression saw Manchester grappling with economic stagnation and labour migration. Liverpool was blamed for charging excessively high rates for importing the raw cotton spun in Lancastrian mills.
Manchester had an idea. If it had its own access to the sea big enough for large ships, it could cut out the middle man of Liverpool. The planned canal infuriated Liverpudlians who tried to ridicule the plans out of existence. But the Manchester Ship Canal was opened in 1894 and the city became Britain's third-busiest port - despite being 36 miles from the Irish Sea.
That was then. With the dissolution of the empire and the introduction of container ships, Liverpool's port became less viable. The withering of the textile industry hit Manchester, and both cities suffered generations of economic decline and depopulation. Poverty was rife in both cities. The nadir was marked by violent riots in 1981 in Manchester's Moss Side and Liverpool's Toxteth districts.
Yet when it came to football and music, both cities punched well above their respective demographic weights, making them special to millions around the globe but also reinforcing and extending the rivalry.
On the pitch, enmities were not clear-cut. Manchester City were the bigger Mancunian club until the Second World War, while Everton were often the pre-eminent Merseyside force. If anything, the relationship between United and Liverpool was one of respect until the 1960s.
Players, such as Pat Crerand, used to watch Liverpool when United did not have a game. "We'd stand on the Kop," recalls Crerand. "The Scousers would have a word with us, but it was good-humoured."
Bill Shankly would call Crerand at home every Sunday morning for a friendly football chat. Shankly and the United manager Matt Busby, who both hailed from Lanarkshire mining stock in Scotland, were also close and Busby had played for Liverpool.
"I always had great respect for Liverpool Football Club and Bill Shankly," Crerand says. "When I go to Anfield I speak to long-standing Liverpool fans who can't put up with what the rivalry has become, with the hooliganism and the nastiness between the fans.
"Liverpool and Manchester are both working-class cities that have produced two of the greatest football clubs in the world. People should be proud of that, but they are not."
The old-time respect did not keep Crerand, in his role as Manchester United's assistant manager in the early 1970s, from snaring the midfielder Lou Macari in the Anfield main stand just as he was about to sign for Liverpool.
By the 1980s, the rivalry was vicious and pernicious, with United manager Ron Atkinson describing a trip to Anfield as like going to Vietnam. Big Ron's experience fighting the Vietcong is unsubstantiated but he can be forgiven for exaggerating; he had just been tear-gassed.
"We got off the coach and all of a sudden something hit us and everyone's eyes went," Atkinson said. "I thought it was fumes off new paint or something, but it was tear gas. In our dressing room before the game there were a lot of fans, Liverpool fans, too, kids all sorts, eyes streaming. Clayton Blackmore was so bad he wasn't able to play.
"I was in an awful state. I'd run in and there'd been two blokes standing in front of the dressing-room door and I couldn't see who they were. I was blinded and I'd pushed one of them up against the wall. Afterwards, [the assistant manager] Mick Brown said, 'What you done to Johnny Sivebaek?' I said, 'What are you on about?' It turned out that Sivebaek, who we'd signed the week before, didn't speak much English and in his first game, against the European champions, he was gassed as he got off the coach and then as he made his way to the dressing room, got hurled against the wall by his new team manager. No wonder he didn't perform that day."
Factions of Liverpool fans frequently sang songs about the 1958 Munich air crash, which killed 23 members of a United travelling party, including eight players. That stopped after the 1989 Hillsborough disaster, which killed 96 Liverpool fans.
At first, United fans barely sang about Hillsborough, but that has changed in recent years. Yet for every United fan who stoops so low, one can find more who respect the continued boycott on Merseyside of The Sun newspaper for its baleful coverage of Hillsborough, and the attempts to get justice for those who perished.
United were top dogs in the 1960s, twice league champions and the first English team to win the European Cup in a decade that saw the clubs have transfer dealings for the last time. Liverpool were far superior to United in the 1970s and 1980s, winning four European Cups and 10 league titles as United went 26 title-free years. But United were usually the better-supported club and matched Liverpool in head-to-head encounters.
For United fans, no matter how dangerous the trip to Anfield became it remained one of the most eagerly awaited matches of the season because of its edge, passion and vitriol.
Liverpool won their most recent league championship in 1990, and their subsequent decline coincided with United's ascendancy under Sir Alex Ferguson. When asked to list his greatest achievement at United, Ferguson replied: "Knocking Liverpool off their … perch. And you can print that."
That was not quite how Scousers intended it to be when they unfurled their "Form is temporary, Class is permanent" banner in 1992 as United squandered a league title at Anfield. United fans are amused that a generation of Merseyside fans have no memories of Liverpool winning the league.
Unlike Liverpool, the Sky TV-led football boom allowed United to capitalise on success and the Mancunians accelerated into a different financial league by regularly expanding Old Trafford. Liverpool, meanwhile, were hampered by Anfield's limited capacity.
United were so successful that many fans objected to the 2005 Glazer takeover of the team, principally on the grounds that they were not needed, while Liverpool fans welcomed their new American owners in 2007 because they were.
That welcome soon turned to protest as promises were broken. Liverpool were taken over by a different American group a year ago and the new owners are far more popular, though they still face the same problems of an Anfield with a restricted, 44,000 capacity.
Both clubs fill their grounds most weeks but Old Trafford now has in excess of 30,000 more seats than Anfield does, allowing United to net £1.8 million (Dh10.4m) more per home match than their rivals. Liverpool need only to look east for their principal reason for building a new stadium or somehow expanding Anfield.
"It's a traditional old English football ground where the crowd are right on top of you. And it's really difficult to get a win there," Scholes says. "When you do, it's brilliant; lose, and you feel down for a few days. United have lost their last three games there, so I'm hoping that run will change [today].
"I respect Liverpool's history and the fact that they have won the league 18 times and the European Cup five times. I hope they respect the fact that we have won the league 19 times."
United surpassed Liverpool's total in May; today's is the first game since then. The 2,000 travelling United fans will gloat, something which will not be quite so easy if their team loses for a fourth consecutive season at Anfield.