Some rivalries in world football need little explanation.
There are those based on geographical proximity, some on the class divide in a nation's society, and others that run along the fault lines of religion and politics.
The contest between Germany's two most prominent clubs - Bayern Munich and Borussia Dortmund - doesn't fit into any of those categories and yet it can be just as intense.
The 'Klassiker' rivalry between Bayern Munich and Borussia Dortmund is set for a new chapter
Meetings between the two sides can often be intense and tempestuous contests
The fans of Bayern will turn the Allianz Arena into a sea of red for one of their biggest fixtures
The yellow-clad Borussia Dortmund fans will travel to Munich in large numbers on Saturday
'Der Klassiker' ('The Classic') is the label now attached to Bayern vs Dortmund encounters but even that is an itinerant description, having been applied to various other German fixtures down the years.
So is it one of the more curious rivalries. Indeed, plenty of people would question whether it's a valid rivalry at all.
But little thought will be given to that in the heat of battle at the Allianz Arena on Saturday night as they fight for crucial Bundesliga points.
As is often the case when these two meet, these points could prove crucial in determining who wins the title. It's especially true this season with the race wide open.
Really, the modern 'Klassiker' has become a rivalry of convenience between the two clubs who have claimed 22 of the last 26 Bundesliga titles.
Bayern hauled in Dortmund to record their seventh consecutive Bundesliga title last season
Dortmund started the current campaign with victory over Bayern in the German Supercup
BAYERN MUNICH vs Borussia Dortmund
Saturday; 5.30pm kick-off UK time
Live on BT Sport 1 and BT Sport Ultimate
Look back over Bundesliga history and you see that Bayern enjoy periods of dominance roughly once every decade - or for nearly an entire decade as is currently the case.
Their other 'Klassikers' down the years have been against Borussia Monchengladbach (1970s), Hamburg (early 1980s) and Werder Bremen (2000s).
Dortmund picked up the baton only in the 1990s as their stature increased with back-to-back Bundesliga successes in 1994-95 and 1995-96, and their 1997 Champions League win on Bayern's home ground.
As Ben McFadyean, president of the Borussia Dortmund Fan Club London, explains: 'It really isn't a traditional rivalry. Dortmund have inherited the role of chief competitor to Bayern from Gladbach, Hamburg, Cologne and Werder Bremen.
'Bayern can be considered the Leviathan of Germany football and various clubs have been the No 2 team - the contenders.
'Dortmund fans consider Schalke as their real rivals. There is just 17km between the two clubs. We say that winning the Revierderby is 'bigger than the championship.'
'But we always want to win at Munich. Stuffing Bayern is every German football fan's dream.
'Dortmund are a mischievous team, a working-class team who are popular right across Germany precisely because they are not Bayern.
'They are seen as a people's club whereas Bayern have lots of what is described as Erfolgsfans, or 'glory hunters."
Borussia Dortmund won the Champions League in Bayern's backyard in 1996-1997
Karl-Heinz Riedle leaps above Bayern goalkeeper Oliver Kahn to score for Dortmund in 1997
Prior to the 1990s, there was for many years an enormous gulf between the two. In the mid-1970s, while Bayern were conquering Europe, Dortmund laboured in the second division.
Indeed in 1971, Bayern thrashed Dortmund 11-1, their biggest-ever Bundesliga win and Dortmund's second worst defeat (they were spanked 12-0 by Monchengladbach in April 1978).
It was only when Ottmar Hitzfeld took over at the Westfalenstadion in 1991 that this sleeping giant awoke. They finished second in his first season, were UEFA Cup runners-up in his second and then won those two league titles.
In 1997, on Bayern's home pitch at the old Olympiastadion, they defeated Juventus 3-1 to win the Champions League for the first and so far only time in their history.
Bayern manager Giovanni Trapattoni (left) and Dortmund boss Ottmar Hitzfeld in a 1997 game
Stephane Chapuisat scored the decisive goal in the Champions League quarter-final of 1998
Stadium: Allianz Arena
Average attendance 2018-19: 75,000
Revenue: £557m (4th in world)
Twitter followers (German): 4.7m
Bundesliga titles: 29
Germans Cup wins: 19
European Cup wins: 5