Without them, stadiums would be empty, atmospheres would be non-existent and the world of football would be a very different place. We are talking, of course, about the millions of fans who turn out every matchday to ensure that sporting arenas around the world are full and buzzing with atmosphere.

At most grounds, fans are segregated according to the team they support, with on-field rivalries matched and often surpassed by those of opposing supporters on the terraces and in the stands. Polarised reactions adorn the faces of fans at either end, and a single goal can result in ecstasy for one group and sheer dismay for the other.

Yet although it is widely accepted that football unites people, many enthusiasts would be surprised to hear that there are several 'official friendships' between fans from various clubs across Planet Football. In Germany alone there are several institutions which have enjoyed cordial relationships with their rivals - both home and abroad - for over 30 years.

European Cup as a platform for friendship
The formation of new alliances may not be as prevalent today as in recent years, but there are still plenty of cases of fan showing their mutual support. This can range from wearing the same scarves, arranging regular friendly matches and various other joint ventures. Supporters often get together to socialise before and after fixtures, while during the game, they frequently celebrate their alliances with welcoming posters, flags and chants.

The friendships often stretch over large distances. Bayern Munich, from south-east Germany, are proud of their associations with northern side St Pauli and Bochum from the west. Meanwhile, 2007 Bundesliga champions Stuttgart of the south-west maintain strong links with Hansa Rostock of the north, while Schalke enjoy excellent camaraderie with the fans of Bavarian side Nuremberg, who are based at the opposite end of the country.

And Germany is not the only nation which boasts such strong supporter relationships, as fans of Paris Saint-Germain's and Celtic will attest. In the mid-1990s, the two sides met in the UEFA Cup Winners' Cup and during the first leg in Paris, an astonishing thing occured between the two sets of fans. The travelling Scottish contingent were so passionate in their support that the bumper PSG home crowd fell silent to listen and admire the chants emanating from the small pocket of green and white.

In the return fixture, the Glaswegians put up a giant banner which read: 'Welcome, our friends from PSG'. The French outfit won the match 3-0, but at the end of the game, all the players received warm applause from the entire stadium. Since then, there have always been Scottish flags visible around the Parc des Princes for PSG home games and numerous fans sporting Celtic shirts.

These same Celtic fans also harbour strong relations with Spain's Villarreal as well as FC St Pauli of Germany, who are so fond of their Glaswegian counterparts that they often arrange charity matches to strengthen their mutual ties. "Our friendship with Celtic is essentially social," St Pauli club official Sven Brux told FIFA.com, while fan's representative Heiko Schlesselmann highlighted the ethic which underpins both groups: "Celtic fans just want their team to do its best. They take pride in the players giving everything for the cause, even if they lose, and not once have they booed their opponents."

The camaraderie between Olympique Marseille and AEK Athens fans can also be traced back to a European tie, when the French faction were so impressed by the fervent support of the AEK crowd that they arranged an official link between the two clubs. In February 2007, a contingent of Marseille fans even travelled to a UEFA Champions League match in the Greek capital to back AEK against the former's domestic rivals Paris Saint-Germain.

You'll never walk alone
Another great example of fans whose goodwill transcends borders is that of Liverpool and Atletico Madrid. Former Atletico star Fernando Torres, currently starring with the Reds, is still adored by fans of Los Colchoneros, and when the two sides met in the Champions League this season, fans from both clubs were singing, dancing and partying together both before and after the game. Incidentally, both matches finished 1-1 and the two sides both made it through to the second round.

FIFA President Joseph S. Blatter once rightly proclaimed that "football brings people and groups together", and it is a sentiment perfectly embodied by one of the most famous terrace songs of the beautiful game: "You'll never walk alone."