It is one of the most volatile and fiercely contested rivalries in English football.
So it is perhaps unsurprising a Leeds school has created a stir by inviting pupils on a trip to watch bitter rivals Manchester United at Old Trafford.
Horsforth School offered parents the chance to send their children to watch the Red Devils in their Europa League tie against Ukrainian side Zorya Luhansk.
The letter has gone viral and infuriated Leeds supporters who claimed the school should be encouraging pupils to support their local club in the community.
The controversial note, penned by PE teacher Mr Steele, gushed: 'With Jose Mourinho at the helm and a raft of summer signings, its exciting times at the Theatre of Dreams.