LONDON - The rivalry between Manchester United and Liverpool, who meet on Sunday, is the fiercest in English football, but the two clubs' periods of success have coincided only rarely.
For all the bitterness that underpins their relationship, United and Liverpool have often displayed remarkable courtesy by stepping aside after gorging on silverware to hand the other club an opportunity to join the feast.
Thus, just as United's recent dominance has coincided with fallow times at Anfield, so the Manchester club spent years in the shadows as Liverpool greedily accumulated trophies in the 1970s and 1980s.
"The dreadful feeling I had as I watched Liverpool winning all those titles is a strong childhood memory," says Gary Neville, a life-long United fan who went on to play for the club for 19 years.
Between them, the two clubs have won 37 English league titles -- with United eclipsing Liverpool's record of 18 in 2011 -- but only five times have they both finished in the top two.
United have reached 18 FA Cup finals and Liverpool 14, and yet they have contested the final just twice, with United prevailing on both occasions, in 1977 and 1996.
The only sustained period of sporting rivalry occurred in the mid-1960s, when Liverpool and United succeeded each other as champions for four years in a row -- Liverpool in 1964 and 1966, United in 1965 and 1967.
That era saw the emergence of Bill Shankly's Liverpool coincide with the blossoming of Matt Busby's all-conquering United, but while Liverpool went on to dominate the next two decades, United had to wait until 1993 for Alex Ferguson to bring the English crown back to Old Trafford.
Ferguson once famously claimed that his finest achievement was "knocking Liverpool off their perch", but by the time United triumphed in the inaugural season of the Premier League, Liverpool were already on the way down.
George Graham's Arsenal had succeeded them as champions in 1991 and it was Howard Wilkinson's Leeds United who had the honor of lifting the old First Division trophy for the final time in 1992.
Second in 1991, Liverpool could only finish sixth in both the following seasons.
For young fans raised on the wall-to-wall hyperbole of the Premier League, the 2008-09 season has provided the only example of England's most successful clubs going head-to-head in the title race.
Rafael Benitez's side enjoyed back-to-back wins over their old foes, including a memorable 4-1 success at Old Trafford in mid-March, but they were pipped to the title by a United team who won 19 of their last 22 games.
Liverpool finished the season just four points off the pace, but four years later, the gap has widened again.
United go into Sunday's game 21 points above Brendan Rodgers' side and Liverpool fan group Spirit of Shankly illustrated the gulf between the teams earlier this week when they branded themselves "a mid-table club" during a protest over ticket prices.
The peculiar synchronicity in the ebb and flow of the two clubs' periods of dominance lies at the root of the rivalry's ferocity.
Where most sporting rivalries thrive on the give and take of even-handed competition, fans of United and Liverpool have each spent decades accumulating enmity while their rivals bathed in glory.
The malice in the rivalry is occasionally heard in the vile songs about the Hillsborough disaster sung by United fans, or the taunts about the 1958 Munich air crash sent in the opposite direction.
The recent feud between Liverpool striker Luis Suarez and United defender Patrice Evra has also provided a focal point for ill-feeling, but in terms of one-on-one battles for silverware, the two clubs have seldom been rivals at all.
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