The Old Firm derby is part of 90min's 50 Biggest Derbies in the World Series.
As is so often the way with football's fiercest rivalries, the Old Firm derby is founded on societal conflicts away from sport.
Dating all the way back to 1888, the United Kingdom's most bitter rivalry is built on divide within the city of Glasgow, with derby day between Celtic and Rangers encapsulating the unbridled hatred each half of the city feels towards the other.
However, it wasn't always this way. In fact, the name 'Old Firm derby' originates from the turn of the 20th century; a time in which both Celtic and Rangers were referred to as 'old firm, friends'. We know, unthinkable right?
When the two sides met in the 1904 Scottish Cup final, having already emerged as the two superpowers of Scottish football, a magazine cartoon took aim at the foundation of the rivalry, suggesting the derby was built merely as a vessel to make money at the expense of local people. The cartoon simply wrote: "Patronise the Old Firm".
However, the political, sectarian and societal conflicts between both sets of supporters soon developed, with Rangers fans associated with Protestantism and British loyalty, while Celtic fans were affiliated with Catholicism and Irish Republicanism.
Yes, these terms may not mean a great deal to some, but they were the fundamental differences that forged such a ferocious rivalry.
Both World War I & II only emancipated things, while 'The Troubles' in Northern Ireland saw tensions peak in Glasgow between the late 1960s and 1990s. Admittedly, this has very little to do with football, but the foundations of the rivalry sustains its bitterness to this very day.
Now for some football.
While Celtic have dominated Scottish football for the last decade or so - due to Rangers' financial difficulties and subsequent demotions - the Light Blues have actually enjoyed the larger share of the spoils since the dawn of this derby. But boy is it close.
The two sides have met 420 times since the late 19th century, with the Gers having won 162 matches to Celtic's 159, while 99 have been draws. The pair have also amassed a staggering 224 trophies between them, with Rangers again edging proceedings, having earned six more pieces of silverware than the Bhoys.
Both Celtic and Rangers have enjoyed significant long-term spells at the summit of Scottish football, as well as having established themselves as serious heavyweights on the European stage - although this tag has been somewhat lost in recent years.
As a result of such competition, the city of Glasgow - as well as the rest of Scotland - has been engulfed by the derby, with the sides often meeting four or more times throughout the season. Celtic Park and Ibrox are absolutely bouncing come derby day, with hordes of police required to keep the peace in an incredibly toxic and hate-fuelled environment.
Fan rivalry is so fierce that Old Firm derbies are often scheduled at midday, or at the latest early afternoon, to try and prevent alcohol consumption from plaguing the derby and endangering the police, staff and supporters alike.
If you think the stadium is hostile on derby day, try going to the pub beforehand.
While Scottish football has never quite had the glowing reputation that English football has enjoyed, both Celtic and Rangers have boasted some of the world's greatest players from past eras.
Whether it be King Kenny Dalglish, Jimmy Johnstone or Henrik Larsson of Celtic, or Jim Baxter, Ally McCoist or Brian Laudrup of Rangers, the pair have boasted some exceptionally-gifted players over the years, and always the lion's share of Scotland's homegrown talent.
It's fair to say that Scottish football may be dwindling in terms of quality, but the rivalry is just as fierce today as ever, especially as Rangers look to topple Celtic atop the Scottish Premiership. While the political and religious divisions in Glasgow aren't as rife today as they once were, neither Celtic nor Rangers can bear to lose the Old Firm derby, and will do everything in their power to claim the enormous bragging rights.
While El Clasico and the Milan derby may be the breeding ground for some of the most aesthetically pleasing football the world will ever see, nothing tops the ferocity and unrivalled chaos of the Old Firm derby.