On Merseyside, crossing the East Lancs Road is the football equivalent of crossing the Rubicon.
As Michael Owen so painfully discovered, once a former Liverpool player signs for Manchester United, there is no going back in fans affections.
It doesn't even have to be a direct transfer. There hasn't been one of them since Phil Chisnall in 1964 .
But even indirect switches are considered beyond the pale.
Is it time to forgive Michael Owen? Reds should forget what he did
Which is what makes The three clubs who have contacted Barcelona over transfer for former Liverpool star Philippe Coutinho that the Old Trafford outfit is one of three clubs who have enquired about unsettled Barcelona playmaker Philippe Coutinho so unsettling.
Reds fans were unhappy when the Brazilian announced his decision to quit Anfield in August 2017, but when he finally did get his wish in January 2018 the blow was softened by the player attracting an enormous transfer fee of £106m up front with £36m in add ons. That allowed the Reds to invest in a world class centre-back in Virgil van Dijk and Coutinho left in relatively amicable circumstances.
In a long emotional post on his Instagram account, Coutinho, who scored 54 goals in 201 appearances for Liverpool spoke of the "unique heart and soul" of the city and how he would "always always cherish the club in my heart. You, the club and the city will always be a part of me."
The post was liked more than two million times.
Kopites still retain a fondness for the magical midfielder, with plenty even hoping for a possible return to Anfield one day.
So would he really alienate Anfield's faithful by signing for United?
He would undoubtedly augment Old Trafford's attacking potential, however Sport, the Spanish outlet which highlighted the latest transfer speculation around Coutinho on today's front page- suggest the South American’s past connection with the Reds could be a ‘stumbling block’ for him to consider joining Ole Gunnar Solskjaer’s side.
They claim this is because the player is not ready to taint his image with Liverpool fans by making the unforgiveable switch if he does leave the Nou Camp.
Such comments will only enhance Coutinho's standing in the eyes of Reds fans.
To paraphrase the lyrics of the old folksong, It's not the leaving of Liverpool which grieves thee, but the prospect of United he will be.
And it sounds like Coutinho has plenty of doubts about that.
