Why do football fans decide to support one club over another in cities where intense, deep-seated rivalries exist?
What makes them choose: family ties, geography, politics, religion, race?
Douglas Beattie spent two years in search of what really lies at the heart of Britain's greatest derbies.
He takes the reader to all the matches with a history of passion and divisiveness which transcends football - making them vitally important to the cities in which they are played.
Liverpool. Glasgow. North London. These local derbies have a history of violence, feuding, social unrest and bigotry.
But what is the truth about their origins, and how does the enmity manifest itself in modern times?
With divisions going back as far as the English Civil War, Douglas discovers why the citizens of Sheffield call each other 'Pigs'; who was Wallace Mercer, the man who divided Edinburgh, and who really eats prawn sandwiches in Manchester.