The Revierderby divide is very much that: a clear division, an unbridgeable boundary between the royal blue and black-and-yellow sides of the Ruhr Valley.
Families can be split in their loyalties though as Holger Leister and his brother Michael are: the former is devoted to Dortmund, the latter swears by Schalke.
Watch: The incredible story of the Leister brothers!
But with Holger requiring a kidney transplant Michael proved blood runs thicker than the Ruhr's waters as he donated a kidney to his BVB-supporting brother
'Reject a Schalke kidney'
"I was going downhill in dialysis, and he said, 'I'll donate you a kidney.' At first I couldn't accept not knowing how his other kidney would hold up. I was scared I'd ruin his kidneys too," explained Holger, whose sense of rivalry — and humour — is still just as strong as the bond he shares with Michael.
"I was initially worried my body would reject a Schalke kidney, but it all went well, and I'm very grateful for it."
Watch: USA stars Pulisic and McKennie put friendship on hold for derby
The pair will now be able to enjoy Saturday's game at the Signal Iduna Park, though they are unlikely to be sitting together on the sofa for what will be a highly-charged 90 minutes.
Regardless of the outcome, Holger will still have a smile on his face.
'Donate if you can'
"My quality of life has improved 60 or 70%. I'm really grateful to him for that, otherwise I'd still be really struggling," he explained, adding he hopes others will follow his brother's example and help save lives.
"For me it's very important. I've been a registered organ donor all my life and I'd recommend to people to find out about it and donate if they can."
The #Revierderby countdown is on â³ #BVBS04