Manchester is a football city. Milan, Munich and Madrid come close, but more people regularly watch the Manchester clubs than the big two of any European city, United averaging 75,000 fans and City 45,000.
In other cities, though, the playing rivalry is more equal: AC and Inter Milan, Real and Atletico Madrid, for instance, regularly accrue silverware while City have not won a major trophy since 1976.
Lacking trophies, City stress their authenticity as the Manchester club in contrast to United's near global reach. Their fans are cast as down to earth, self-effacing, humble Mancunians who sneer at the United supporters in Trafford (technically a different borough in the Greater Manchester area and therefore not in the city of Manchester) who only started watching the club when they won the 1999 treble of Premier League, FA Cup and Champions League.
United fans counter that most City fans are from Manchester's satellite towns like Rochdale and Stockport and did not need a passport until recently because their team never plays abroad. They have heard so many new City signings refer to the club as "massive" that they have hijacked the phrase in mockery. The terms "bitter blue" and "obsessive" are frequently used.
Before World War II, City were the bigger team in Manchester. United, playing as Newton Heath, may have won the first Manchester league derby 5-2 in 1894, but aside from a spell when a Billy Meredith-inspired side won two championships and an FA Cup between 1908 and 1911, City were better supported.
They won the FA Cup in 1904 and 1934, the league title in 1937 and were runners-up in 1904 and 1921. City's crowd of 84,569 for an FA Cup tie with Stoke City in 1934 remains a British record for any game outside London and Glasgow.
United, meanwhile, came within a game of being relegated to the third division in 1936.
Meredith, the Welsh superstar, was the biggest name to play for both clubs before the war and still holds the record for the most derby starts: 14 for City, 15 for United, two more than Sir Bobby Charlton. City's Joe Corrigan ended on 26, Ryan Giggs is on 25 (though he came on as a substitute in another eight).
Meredith joined City in 1894 and spent 11 years there until his first spell was terminated in 1905 as a result of a bribery scandal.
After serving a suspension, he signed for United aged 32. In fact, United's successful team of the first decade of the 20th century was built around players who City fans will tell you were "stolen" from the blue side of Manchester.
Forbidden from playing for City, they were to be sold at auction until United "acquired" them at a bargain rate. Implausibly, Meredith stayed at United until he was 47, before City re-signed him. The forward defied time and appeared in the 1924 FA Cup semi-final for City in his 50th year.
Between the two World Wars there were precious few league derbies - in fact in those 20 seasons, United played City in only five of them.
From those 10 games, United won only two, drew two and lost the rest, including being battered 6-1 at home in 1925/26.
It was only after United appointed Matt Busby, a former City player, as manager in 1946 that the tide began to turn. Playing at Maine Road for four seasons after German bombers had blitzed Old Trafford, United finished runners-up in the league four times in five years and won the FA Cup. And that was before the "Busby Babes".
The Munich air crash in 1958 decimated that side - eight players died while two more never played again - but United benefited from a wave of emotion which became support.
Brian Kidd, who would play for both teams, is proof of this. He wore City colours until he switched to United following the tragedy.
Two days after Munich, City's players wore black armbands and a minute's silence was held as a mark of respect.
In their first match-day programme after the crash, Alan Douglas, the City chairman, wrote: "For many years there has existed a rivalry between the two clubs, but just as we at Maine Road have rejoiced in United's many triumphs, so now we share their sorrow ... We at Manchester City are convinced that United will recover ... if we can do anything to help them in any way, however small, to achieve that objective, we shall regard it as a privilege to do it."
Fifty years later, City fans were impeccable in observing another moment of silence to remember the Munich dead - though elements of their support persist in singing songs about the disaster.
In the 1960s, United began to assert their pre-eminence, although Joe Mercer's fine City team featuring Colin Bell, Francis Lee and Mike Summerbee pipped United to the league title in 1968. Even then City fans had little time to revel in the attention: United won the European Cup a few weeks later.
City became the dominant Manchester team thereafter, winning the League Cup and the European Cup Winners' Cup in 1970.
As United slipped into a decline that would culminate in relegation in 1974, the derbies remained heated. In December 1971, City beat United 4-1 at Old Trafford in a game infamous for George Best's poor tackle on Glynn Pardoe, the visiting full-back, which shattered his leg. Pardoe later said: "It was a shocker. It was touch and go whether or not the leg would have to be amputated."
But the derby of March 1974 will go down as the worst for player violence, with Clive Thomas, the referee, leading the teams off the field following an on pitch bust-up after City's Mike Doyle and United's Lou Macari were dismissed. Macari and Doyle, who comes from a family of City supporters and who always made his dislike of United clear, both refused to leave.
Six weeks later, City applied the coup de grace, winning 1-0 with a famous back-heeled goal from Denis Law in front of the Stretford End where he had been proclaimed king for years by United fans.
"My last goal for City against United in 1974 did nothing for the adrenalin," Law said.
United fans invaded the pitch with six minutes to play but the result stood. Law is often credited with relegating his former side to the English second division, but United would have gone down regardless as results went against them.
City won the League Cup in 1976, beating United 4-0 along the way in a game which saw Bell's career all but ended after a tackle by Martin Buchan. While subsequent City successes have been rare, United have an uncanny knack of upstaging their triumphs.
Even when City won promotion from the third division in 1999 in a thrilling game at Wembley in which they scored twice in injury time to level the scores against Gillingham, United's Champions League final win over Bayern Munich dominated headlines.
City fans point to several derby wins, the most notable being "the 5-1" of September 1989 when United's expensive new signings were humiliated by a City side seemingly short on quality. Sir Alex Ferguson, the United manager, said after that game that he went to bed hoping never to wake up, but has since overseen convincing victories over City, revenge coming when an Andrei Kanchelskis-inspired United side beat City 5-0 on Bonfire Night 1994, with the Russian winger bagging a hat-trick.
In 2001, Roy Keane was sent off for a vicious, premeditated tackle on City's Alf-Inge Haaland.
The Norwegian had accused Keane of faking injury in a match against Leeds United in 1997, when actually the Irishman had snapped his cruciate knee ligament. Keane bided his time for revenge. The United captain did not even wait for the referee to show him the red card and the challenge contributed to Haaland retiring through injury.
And yet despite their proximity, City were seldom seen as United's biggest rivals, that honour accorded to Liverpool by fans, Chelsea and Arsenal by the players.
That is changing. New ownership at City has allowed the club to compete in the transfer market for the best players in the world.
Last season's derby games, including two cup semi-finals, were among the highlights of United's season and boasted an atmosphere missing in derbies for years.
Tonight, with City being title challengers again, the whole world will be watching.