Morbo is the word, as they say in Spain. Trouble is, it's one of those awkward ones that defies easy translation. No matter how you try, you can't quite nail the word down. Don't bother with a dictionary, for it will only confuse you further, the word having other meanings that are not applicable to football.

Most treat it as a noun and translate it as something like 'disease', which is hardly appropriate to this context. Nevertheless, since no history of Spanish football can pretend to be complete without it, the word has to be confronted. Take Real Madrid and Barcelona for starters - the easiest introduction to the idea and to Spanish football in general. There is so much morbo festering between these two sides that they would have to employ a very powerful priest to exorcise the phenomenon, always presuming that they wanted to.

It's not merely that they hate each other with an intensity that can truly shock the outsider, but that each encounter between them always has a new ingredient. This is the essence of morbo. It feeds off itself and keeps growing until it becomes a self-regulating and self-perpetuating organism.

Spain is something of an unforgiving country - one that, as the writer Jan Morris once put it, dedicated itself to the art of the self-inflicted wound and that is committed to acts of cruelty against its own kind rather too often. While there are people still around who fought and suffered in the civil war and whole swathes of the populace who lived under Franco and either hated or loved every minute of it, you have a recipe for a very spicy soup indeed.

The cat-and-dog synergy that has always existed between Barcelona and Madrid, even before the Catalans began to get assertive, is the basis of the morbo , but the Spanish are most adept at stoking and fanning the flames.

The football tabloid Marca is the unrivalled expert in the art of creating the new season's spice between these two great clubs. Based in Madrid, although it originated in the Basque Country, it makes no secret of its allegiance to its local heroes, nor of its antipathy towards the Catalans in general and FC Barcelona in particular.

In July 2000, when the Portuguese midfielder Luis Figo became the world's most expensive player on being transferred from Barcelona to Real Madrid, the paper went into a frenzy. The transfer was perceived by the Barcelona faithful as an affront to their image and their soul. There, were few precedents.

Only the Dane Michael Laudrup and Bernd Schuster in recent years had dared to make the move and while Laudrup, like Figo, was popular with the supporters, he had fallen out publicly with Johan Cruyff, then Barcelona's manager. Schuster's move in 1988 was greeted with derision in the Catalan capital, but few were sorry to see him go. Figo, by contrast, had the Barça faithful in his pocket, was at the height of his fame and appeared to be another convert to the Catalan cause.

When he returned to the Camp Nou on 22 October 2000, three months after what was probably the most notorious transfer in Spanish football, Marca , proud of its reputation as a troublemaker, published a simple picture of Figo's ear under the headline Figo: Te van a calentar la oreja (Figo, they're going to make your ears burn). And they did, Figo, famed for his consistency, was unable to function.

Every time he ventured near a corner flag he was met with a barrage of abuse, angry gestures and various objects - including three mobile phones, several half bricks, a bicycle chain and a shower of coins, all meticulously noted in the referee's report. (Figo will not be playing in Tuesday's first leg because he is suspended.)

Three years earlier Luis Enrique, one of the most consistent and affable Spanish players of the 1990s, moved from Madrid to Barcelona. Marca - just in case the player's appearance was not going to be greeted with sufficient abuse - decided to remind the Spanish public the day beforehand that he was a traitor. 'And we all know what happens to them,' they added.

Unlike Figo, Luis Enrique scored when the two sides met, and has subsequently continued to add value to his own particular morbo quotient by such simple acts as kissing the Catalan shield on his shirt whenever he scores against his former club (he is not a Catalan - no matter) or telling the press how happy he is to live in such a vibrant and open city as Barcelona - the implication being that Madrid is exactly the opposite.

Laudrup moved the other way in 1994 and ended Barcelona's so-called Dream Team sequence of four consecutive titles by linking up instinctively with the Chilean forward Ivan Zamorano and helping to return the title to the Bernabeu in his first season there. A more decent and modest chap than Laudrup it would be hard to find, but his reception at the Camp Nou that season was so astonishingly hostile that the player, like Figo six years later, simply could not function during the game.

All cultures must possess their version of morbo , but in Spain it has been the driving force behind the public's relationship with football for 100 years.

In 1902, FC Barcelona were three years old and Madrid FC, as they were then called, a year younger. Alfonso XIII, already a patron of the fledgling game, was to be crowned that year, and Carlos Prados, the new president of the Madrid club, hit upon the idea of organising a football tournament to coincide with the coronation's festivities in the capital. Five teams turned up: Vizcaya (the name of the north-western region of the Basque country), Barcelona, Madrid, New Football de Madrid and the other team from Barcelona, the provocatively named Español - the club did not change its name to the Catalan spelling (Espanyol) until the mid-1990s. The tournament was to feature the first recorded game between Barcelona and Madrid, the Catalans drawing first blood with a 3-1 win. Vizcaya eventually beat Barcelona 2-1 in the final.

The interesting detail for morbo watchers was the fact that the organisers, probably miffed that the final had been disputed by the Catalans and Basques, decided to cobble together a quick concurso de consolacion (third place play-off) the same afternoon, won by Madrid of course.

The newspapers used the word trofeo to define what Madrid picked up that afternoon, although it must have been something hastily brought along from somebody's personal silver collection, because Vizcaya had already been given the official trophy - a cup that was to become the King's Cup (Spain's equivalent of the FA Cup) the following year and that would be thereafter presented to the winning team by royal hands. Madrid's reputation as poor losers seems to have been born immediately.

It took the two teams three years to get around to playing each other again, this time in Barcelona. The Catalans won again, 5-2, and, according to a report from the time, in the rather posh restaurant Francia, where the hosts entertained their guests after the game: '...certain rather unfortunate and unhappy references were made by the visitors from Madrid as to the alleged unsporting nature of the encounter, the partial nature of the referee's decisions and the indelicate phrases used by certain sections of the spectating public, thus bringing into some disrepute the nature of this sporting event.' So began a century of mutual antipathy.

Not only is the dislike between the two clubs an interesting phenomenon in sporting terms, it also has implications that stray deeply into the sociology and politics of the country. It might not be going too far to say that the strife and struggles between the two clubs from 1905 onwards accurately mirror the very essence of twentieth-century Spanish history. The two cities have always been moving in different directions, partly through bloody-mindedness, partly through political allegiance, but mainly through clear cultural differences.

A supporter of Real Madrid seems a very distinct creature from a supporter of Barcelona, a fact that cannot be attributed wholly to the fact that they probably talk about football in a different language. Madrid is a bourgeois, grand, rather suffocating sort of city on first acquaintance. The surrounding countryside is bleak and bare, - suggestive of some harshness in the citizens. To an outsider it is not a welcoming city - its taxi drivers grumpy and sullen, its waiters coldly efficient, its shops too self-consciously trendy. Madrid was built on and is sustained by the notion of centralisation - in this century exemplified by Franco's obsessive opposition to regional nationalism, which he regarded as one of the principal reasons for the turmoil of Spain's ill-fated second republic.

Madrid was symbolically in the geographical centre of the country, put there by Felipe II in the mid-sixteenth century. It is part and parcel of the Madrid-Barcelona morbo that the latter seems to inhabit a different planet. Despite the fact that Madrid has the Prado, the seat of government and the royal family, according to John Hopper's book The New Spaniards almost all the ideas that have shaped Spain's modern history - republicanism, federalism, anarchism, syndicalism and communism - have found their way into Spain by way of Catalonia.

Fashions, whether in clothing, philosophy or art, have tended to take root in Barcelona's more welcoming soil years before they gained acceptance in Madrid. The whole city seems to be up to something and it seems to like itself, not in a narcissistic way but rather in a confident, breezy manner that conveys a sense of ease with itself. Barcelona fans labour under the touchingly innocent belief that everyone else in the world, apart from Real Madrid and Espanyol fans, is happy to accept that their club is the biggest on earth. When you first step into the Camp Nou and take your seat, the pretension begins to look a little more justified, for it is indeed an astonishing sight.

The whole place seems so taken up with its own socio-cultural significance. No wonder Madrileños hate it so much. The Bernabeu is a more hostile place, as beautiful as the Camp Nou but more brooding somehow, more edgy. It has a we-don't-care-if-you-hate-us look in its eye. Barça fans will ask you where you are from, what you think of their team, what you think of the city, who you support, and so on.

When their travelling supporters visit other Spanish cities there is rarely trouble. They have four morboso encounters a season, two each with Madrid and Espanyol, and apart from a smidgen of nastiness between themselves and Atletico Madrid, they cannot believe that there is any more of the stuff to be found in the rest of the country.

Of course, the history of Spain has served to drive the two cities apart. Franco banned the Catalan language and all public use of it, even encouraging people through public notices in telephone booths that they should communicate, when the pips stopped pipping, in Castilian, or 'Christian language' as his administration called it.

As with the Basques, this legislation was, to say the least, unpopular but due to the efficiency of the murderous police state that Franco subsequently put into operation, the policy worked, on the surface at least. Franco's decision to remain technically neutral in the Second World War meant that football could continue, subject to a certain degree of political manipulation and tampering.

By and large, in the early years of Franco's reign, people who were opposed to him, decided that discretion was the better part of valour and kept a low profile, if they had not already fled into exile, which meant that games between Barcelona and Real Madrid could continue, albeit on a slightly more anodyne basis. Nevertheless, our old friend morbo was never very far away. By 1943, the Copa del Rey , the King's Cup, had been renamed the Generalisimo's Cup, in Franco's honour. The previous season Barcelona had the temerity to win, though they had struggled in the league - a failure attributed by those brave enough to speak out to a widespread bias in the refereeing.

More likely, the referees knew which side their bread was buttered on. It might well have been true that certain officials had been receiving specific instructions (and payment) to act against the Catalan club, but Franco was probably cleverer than that. To take away all the morbo from the encounters between the clubs at one fell swoop would hardly have served his purposes.

By allowing Barcelona the occasional triumph, if that is indeed what he did, he ensured that the pre-eminence of Real Madrid and the symbolic light that it shone on his regime were assured in a more authentic, more dangerously convincing way.

But, for the conspiracy theorists, the semi-finals of the cup in 1943 make very juicy reading, for it would have been taking the morbo just a little too far, as far as the Falangists were concerned, if Barça had been permitted to win it two seasons running, especially at that delicate stage of the Nationalists' gameplan.

These events in the dressing room ensured that every subsequent encounter between the two sides would be forever burnt by the flames of morbo . The first game, at Barcelona's old stadium Les Corts, ended in a 3-0 win for the Catalans.

The victory was somewhat pyrrhic, however, since Jose Escola, Barça's star of the early 1940s, was taken off on a stretcher in the first half with José Maria Querejeta's stud marks on his stomach. Certain sections of the crowd beginning to berate Madrid's players and the leniency of the referee acted as a red rag to the bull of the Madrid-based press, which toed Franco's line whenever necessary.

The newspaper Ya (Now) reported that there had been ungentlemanly whistling and booing and that this had been symptomatic of a 'clear intention to attack the representatives of Spain'. Say no more. In the return match in Chamartin, Madrid's then ground, Barcelona were defeated 11-1, a score that is still a blot on their statistical history. The fact that the Catalans had a player sent off before half-time hardly seems a sufficient explanation for the rout.

Perhaps it had more to do with the fact that the director of state security went into the their dressing room before the game and allegedly told the assembled gathering that some of them - despite their lack of 'patriotism' - were only playing because of the regime's generosity in permitting them to remain in the country.

The fact that Real Madrid's official centenary book generously describes this game in 1943 as ' El baño del siglo ' (The stuffing of the century) illustrates better than anything else the unremitting hostility that exists between these two great sides. It all adds to generate this sense of morbo , which will suffuse Madrid and Barcelona, and the rest of Spain, over the next 11 days.

· You've read the piece, now have your say. Email your comments, be as frank as you like, we can take it, to the football.editor@guardianunlimited.co.uk or mail the Observer direct at sport@observer.co.uk.