It is the last great footballing divide, the most lingering transfer embargo. No player has moved directly from Liverpool to Manchester United in 77 years, since Allenby Chilton in 1938.
And it has been almost as long since anyone went the other way. It was in April 1964 that Bill Shankly paid £25,000 to secure the signature of United's Phil Chisnall.
In the 51 years since then, players have moved between Tottenham and Arsenal, Celtic and Rangers, Barcelona and Real Madrid. Carlos Tevez, Terry Cooke, John Gidman, Wyn Davies and Denis Law have all switched allegiance between Manchester United and Manchester City.
There have even been players who have turned out for both Norwich and Ipswich.
But between Liverpool and United there has been nothing at a senior level (though Scott Wootton - these days plying his trade at Rotherham - took up a place in United's academy aged 16 in 2007 after turning down an offer to remain at Liverpool's).
Sure, Peter Beardsley, Paul Ince and Michael Owen all played for both clubs. But they allowed a suitable diversion between engagements. There are some histories, it seems, reckoned unwise to trample across.
As Gabriel Heinze discovered in 2007. United agreed to allow the Argentinian to seek a move from Old Trafford after he became disillusioned. But when it was discovered that his representatives had been in discussion with Liverpool manager Rafael Benitez, Heinze was told in no uncertain terms that he could go anywhere but there.
United's then manager Alex Ferguson felt that the club's supporters would never forgive him assisting the team building of their most significant rivals.
Heinze was sold elsewhere. Though given he ended up with Real Madrid, he cannot have been too disappointed.
It is not that the two clubs do not covet each others' resources. There have been players on either side both sets of supporters would have loved to have seen in their own ranks.
Ferguson himself was said by those who played under him to be almost comically obsessed by Steve McManaman.
Roy Keane recalls that every match against Liverpool would be preceded by the manager's repeated insistence that the only way to stop them was to stop McManaman.
Yet, however much he admired him, Ferguson never made a bid for the winger. Just as he never did for Steven Gerrard or Jamie Carragher or Fernando Torres.
This despite the fact that he was a firm believer in Shankly's two-pronged first rule of transfer business: make sure you weaken a rival at the same time as you strengthen your own hand.
Perhaps because of his upbringing in Glasgow, Ferguson recognised that there are some cultural divisions that cannot be quickly or simply breached.
A student of psychology, he understood that a player's most important muscle is between his ears. Becoming the target of the inevitable vitriol that will pour in their direction is not an easy burden to carry; few could flourish in such circumstances.
And it is a vitriol as likely to arrive from their new supporters suspicious of their past allegiance as from the old, angered by their defection. Not every player has the mental strength of Sol Campbell or Luis Figo. In may be easier simply to shop elsewhere.
History suggests Ferguson had a point. Certainly for Chisnall himself the move was not one garlanded in rosettes. Although he was transferred at a time when the Liverpool/ United rivalry was nothing like as scabrous as it has latterly become, things did not work well for him.
He did play in Liverpool's first ever European game - an 11-1 aggregate victory over Knattspyrnufélag Reykjavíkur of Iceland in the European Cup. But that was just one of six first team outings he made for the first team.
After three years of underachievement he moved to Southend in 1967, before retiring to work in a malt loaf factory in Urmston. It is not the kind of future Raheem Sterling is presumably anticipating. (© Daily Telegraph, London)