London - When Liverpool and Manchester United clash at Anfield on Saturday, one of the Premier League's most explosive rivalries will serve as a referendum on the current state of both clubs.
While in-form United hope to mount a sustained challenge for their first English title since 2013, Liverpool, already seven points behind their second-placed rivals, are battling to stay relevant after an inconsistent start.
Over the last four decades, United and Liverpool have rarely been in the ascendency at the same time, so there is extra pressure on Jurgen Klopp's side to prove they can keep pace when Jose Mourinho's men arrive on Merseyside.
While both teams play in red and come from cities separated by just 31 miles, beneath the superficial similarities lies a bitter feud.
Here AFP Sport sets the scene for the latest instalment:
Best of enemies
Formed as Newton Heath in 1878, Manchester United were born 14 years before Liverpool emerged in 1892, and for a brief period, the clubs had a surprisingly cordial relationship.
In 1915, players from the teams even joined forces to fix their match to ensure United avoided relegation, with four from Liverpool and three from Manchester subsequently banned for life.
As late as 1958, Liverpool offered to lend players to United after several of their stars were killed in the Munich aeroplane crash.
But the friendly rivalry had turned nasty by the late 1970s as hooligan gangs fought bloody battles, while United were sprayed with tear gas by a Liverpool fan when they stepped off the coach before a match at Anfield in 1986.
There hasn't been a transfer between the clubs since Phil Chisnall moved from United to Liverpool in 1964.
The enmity plumbed new depths when Liverpool striker Luis Suarez was suspended for eight games after racially abusing United's Patrice Evra in 2011.
Liverpool enjoyed one of English football's most dominant periods in the 1970s and 1980s, a golden era which saw them win 11 of the club's 18 league titles, lift the FA Cup three times and win the League Cup four successive years.
Not content with hoovering up the domestic prizes, Liverpool reigned supreme abroad, winning the European Cup four times and the UEFA Cup twice.
But Liverpool's preeminence had ended by the dawn of the Premier League as United rose to power.
Between 1992 and 2013, United were crowned champions 13 times, surpassing Liverpool's record as they took the club's overall haul to 20 English titles.
With 12 FA Cup triumphs, United are five ahead of Liverpool, but it remains a source of angst at Old Trafford that the team's three European Cup/Champions League victories fall short of the five on display at Anfield.
"It doesn't matter if we are playing tiddlywinks, when we get together you expect sparks to fly," former United manager Alex Ferguson once said of the Liverpool rivalry.
Ferguson did more than most to light the fuse, complaining that visiting managers left Anfield "choking on their own vomit" after an acrimonious 1988 clash and crowing that he had made it his aim to "knock Liverpool from their perch" when he arrived in Manchester.
Liverpool managers were just as susceptible to the passion play, with Rafael Benitez infamously embarking on a heated press conference rant about the "facts" that showed Ferguson and United were given favourable treatment by the referees and the governing bodies.
There wasn't always a spiteful relationship between the custodians of the clubs as legendary United boss Matt Busby and Liverpool great Bill Shankly were good friends at the height of their powers in the 1960s.
While the football feud has gradually come to the boil, there was never any love lost between the cities, a loathing that had its origins in Manchester authorities building a ship canal in 1877 that threatened Liverpool's ocean-going trade.
When the Beatles took the charts by storm in the 1960s, Liverpudlians trumpeted Merseybeat as the country's greatest pop export, a boast Mancunians used themselves 20 years later when their city produced touchstone bands like the Smiths, the Stone Roses and the Happy Mondays.
Liverpool supporters claim they beat Manchester to the craze of dressing in fashionable 'casual' sportswear that swept the terraces in the 1980s.
But the rivalry has often turned toxic, with Liverpool fans chanting about the Munich crash and their United counterparts making sick songs about the 1989 Hillsborough disaster that killed 96 Reds supporters.