WHEN MARTIN Palermo scored his 180th goal for Boca Juniors last autumn, the club celebrated by releasing a series of 180 pairs of commemorative boots, each inscribed with the details of one of his goals. The pair that fetched the highest price was number 73, the so-called "limp goal" he scored against River Plate in the second leg of their Copa Libertadores quarter-final in 2000.

He had been out for six months with a full cruciate tear of his right knee and palpably wasn't anywhere near full fitness, but insisted on being included on the bench. The River coach, America Ruben Gallego, was sceptical and said that if Palermo played, he would pick the long-retired River legend Enzo Francescoli. Nonetheless, with Boca 2-0 up on the night and 3-2 up on aggregate, Palermo came on with 15 minutes to go and sealed the win with what the 34-year-old still describes as "the most emotional goal of my career".

It is that kind of moment that makes the superclasico so special. Should Boca beat River at home this evening, they could close to within a point at the top of the table, but that is of secondary concern. As Alberto J Armando, Boca's great chairman and the man after whom the club's ground, popularly known as La Bombonera, is officially named, said to the Brazilian forward Paulo Valentim after signing him in 1960: "Don't worry about the other games, just score in the superclasicos". On Wednesday, Boca beat Cruzeiro in the first leg of their Copa Libertadores last 16 tie - far more convincingly than the 2-1 scoreline may suggest - yet most of the chants were directed at River. "On Sunday," the most common went, "we're meeting the Chickens."

River have been mocked as "gallinas" - chickens - since 1966, when they threw away a two-goal lead to lose to Penarol of Uruguay in the Libertadores final. That was only the most painful of a series of chokes that saw them fail to win a league title between 1957 and 1975, but 11 times finish as runners-up. "Finally," the Uruguayan midfielder Gabriel Cedres said on leaving River for Boca in 1997, "I have got rid of the feathers."

The term has been appropriated to an extent, but it is still considered sensitive enough that the Manchester United forward Carlos Tevez, then at Boca, was sent off at River's El Monumental for miming the flapping of chicken wings after scoring a vital late goal when the sides met in the Libertadores semi-final in 2004.

On Wednesday, River lost away in the Libertadores to another Buenos Aires side, San Lorenzo, but the great talking point of the game was the comment of San Lorenzo's former River winger Andres d'Alessandro that he would rather Boca won on Sunday.

His point was that a Boca victory would demoralise River ahead of the second leg, but it was treated as some kind of great betrayal.

The other source of discussion this week has surrounded the appointment of referee Gustavo Bassi who, slightly foolishly, has let slip that he is a Boca fan. Then again, given that most of the country falls on one side of the Boca-River divide or other, it would be hard to find anybody who didn't have some leanings one way or the other.

This is a fixture that possesses an acute sense of its own history, and lives with one eye constantly on posterity. Two years ago, for instance, River fans released thousands of orange balloons at the Bombonera to mark the 20th anniversary of a famous goal scored by Norberto Alonso in a superclasico played, for reasons of which no-one seems quite sure, with an orange ball.

Ruben Sune, meanwhile, may have won eight league titles as a forward with Boca, but he remains haunted by a penalty he missed in a superclasico. "It was the saddest day of my mother's life," he said. "We lost the game 5-4 and when I missed my first thought was for my family, most of whom are bosteros."

Bosteros are Boca fans, the name deriving from bosta - shit - because the stretch of river that passes near the ground is where the city's sewers are emptied. Now that the term - like gallinas - has been appropriated, River fans prefer to use the term puercos - pigs - to describe their rivals.

Yet, River themselves originate from the same area. Both clubs were founded in Boca in the early 20th century when most of its population were Italian immigrants, but, in 1923, River moved to Nunez, a far wealthier district.

As they began to draw fans from the middle-classes, their spending increased so that by the late 1930s they were known as los Millonarios' (the millionaires). Boca, by contrast, remained the team of the people, something celebrated by fans in their chant, "Boca es el pueblo, el carnival".

Even their styles of play are supposed to reflect that background. River's greatest team was "la Maquina", the side managed by Renato Cesarini that, with its fabled front five of Felix Loustau, Angel Labruna, Adolfo Pedernera, Jose Moreno and Juan Carlos Munoz won three league titles between 1941 and 1945.

"You play against la Maquina with the intention of winning," said Ernesto Lazzatti, Boca's centre-half at the time, "but as an admirer of football, sometimes I'd rather stay in the stands and watch them play." Yet, the other two championships in that period were won by Boca, whose strength was said to be their "garra" - literally "claw", but the term means spirit or, as a more recent chant puts it, "huevos" - balls.

The myth, of course, is far from the full truth: no team with the glorious anachronism that is Juan Roman Riquelme in its ranks could ever be said to be devoted merely to battling spirit, and nor is it true that River, supported, it is said, by 30% of the Argentinian population, or Boca, supported by 40%, draw their fans from only one social class, but the stereotypes persist.

Come this evening, it will again be working-class against middle-class, pigs against chickens, and another chapter of the superclassico's history will be written.