Celtic and Rangers have exerted a vice-like grip on Scottish football for a century but their bitter rivalry is underpinned by sectarian tensions that predate the existence of either Glasgow club.
The ferocious passion generated when the teams clash in the Old Firm derby makes global headlines but behind it is a hatred whose roots can be found in Scotland's history of religion and immigration.
The Scottish reformation movement of the 16th century saw the previously pious Catholic nation of Scotland adopting Protestantism as its national religion.
Religious hostilities existed for the next 300 years. Tensions were further stoked by an influx of both Catholic and Protestant immigrants from the north of Ireland during the 19th and early 20th century.
Duncan Morrow, a lecturer in politics at the University of Ulster, was asked by the Scottish government in 2011 to chair the independent Advisory Group on Tackling Sectarianism in Scotland.
"Irish immigration to the industries of central Scotland brought with it sectarian rivalries and stirred a cultural anti-Catholicism, and even anti-Protestantism, that infected the workplace, local politics and the shape of social life for generations," he told AFP.
"Rivalries develop between football clubs everywhere but the big difference in Glasgow was the clubs associated with this sectarian rivalry."
Celtic were founded in 1887 by a Marist (Catholic) Brother from Ireland and the club's origins were firmly embedded in Irish Catholicism.
Rangers, founded in 1872, became the team of the Scottish Protestant working class almost by accident.
At first, relations between the two clubs were cordial, with Rangers supplying the opposition for their neighbours' first game.
- No Catholics -
However, the opening of a huge shipyard in Govan, just a short walk from Rangers' Ibrox home, by industrial firm Harland and Wolff, saw an influx of workers from Belfast in the 1910s. The company's infamous "Catholics need not apply" policy soon spread to the club their workforce adopted.
The clubs' identities were shaped in direct opposition to each other, with Celtic associated with socialism and Irish Republicanism, Rangers with conservatism and Northern Irish unionism.
This was reflected in the songs the clubs' supporters adopted, which have tended to use the political events being played out in Northern Ireland as their inspiration.
The signing of former Celtic player Mo Johnston in 1989 brought Rangers' boycott of Catholic players to an end.
For a time it seemed as if the religious-inspired rancour was being pushed to the margins in their rivalry in a more secular society.
But the rise of social media, opposing views over the Scottish independence referendum and Rangers' 2012 liquidation have led to heightened tensions in recent years.
"In a time where religion is less important in society it is almost as if it has become part of the identity of football in Scotland," said Morrow. "In a sense, sectarianism now is a way of behaving rather than a way of believing."
Morrow added there is evidence that violence spikes when sectarian rituals are played out in the street or at stadiums.
"If you speak to young people, if you look at social media and visit some places in central Scotland then you can see that sectarianism is still there in communities," he said.
It is an issue that former Celtic manager and player Neil Lennon can relate to after enduring sectarian abuse during his career in Scotland.
In 2002, a year after signing for the Hoops, Lennon, a Catholic, stopped playing international football for Northern Ireland, where the majority of the population are Protestants, following a death threat.
In 2011, while manager of the Parkhead club, he was sent parcel bombs and bullets in the post and when he quit in 2014 he said he had become worn down by the abuse.
But speaking after the Old Firm fixture at Ibrox in March, where sectarian singing was heard and fans clashed in Glasgow city centre, Lennon said he believes the situation is improving.
"In my opinion it has got better but maybe that's because I'm out of the Glasgow goldfish bowl," the current Hibernian manager said.
"There is no place for sectarianism any more. Some people use football as a vehicle to vent their frustrations or beliefs. But not many people want to listen to it any more."
© 2018 AFP