There are huge derby matches all over South America. The history of the continent's football is based on city rivalries. Unlike, say, Spain and most of Italy, the big rivalries are not between different regions of the country, but between different teams from the same city.
So why does Boca Juniors vs. River Plate stand out?
There are two explanations. One is a consequence of the importance of Argentina to Latin American football. The game was introduced, especially in the south cone, by the British. But it was Argentines and Uruguayans who did much to carry the sport north. And since Argentina is so much bigger than Uruguay, the entire region tended to take its cues from there.
Before he became "Che," a young medical student made an early 1950s trip around Latin America. The mere fact that he and his friend were Argentine was sufficient to install them as football specialists as they made their way north. Other countries have embraced much of Argentine fan culture. And as Boca-River is Argentina's biggest derby, its importance was guaranteed.
The second explanation has to do with the nature of the rivalry itself.
Both Boca and River grew up as neighbours, in the dockland area of Buenos Aires. Boca have stayed put. River, meanwhile, long ago achieved the immigrant dream of moving out to the snooty suburbs.
It gives the two old rivals clear and contrasting identities. The Boca area is colourful, full of character but run down. Its streets are narrow and cramped. For this reason the club's stadium is known as the Bombonera -- the chocolate box. It is built straight up. Not for vertigo sufferers, Boca's ground pulses pure passion, with the crowd intimidatingly close to the action.
Out in Nunez, River's neighbourhood is all wide avenues. There is plenty of space. The corridors are as so wide they could be in the United Nations building. In a giant bowl, the fans are far from the pitch. Compared with the warmth of Boca, the ground feels cold, often literally, with a wind blowing across.
Both clubs moved into their legendary homes just before Argentine football entered its 1940s golden age. They are, then, well defined symbols. Sociological studies reveal that the fan base of the two clubs is broadly similar. But in terms of image, of what they appear to stand for, they could hardly be more different. Boca are a celebration of working class sweat and solidarity. River are the more cool and cerebral bourgeoisie.
This fault line -- the haves and the have-nots -- is present in many of the big South American derbies. So it proved easy for the rest of the continent to identify with Boca-River, and boost still further the prestige of the Buenos Aires Superclasico.