Two of the keys for Champions League finalists Real Madrid and Juventus, Marcelo and Dani Alves, are the best examples of the importance and emergence of the attacking fullback.

Gianluca Vialli, the decorated former Italian international, had a theory that the right back was invariably the worst player in a team.

If you were left-footed, a scarcity value meant coaches tended to leave you alone. If you were right-footed and skillful, you would be played as a winger. If you were right-footed and good defensively, you would be moved into the center. The right back was the player left over when the other positions were filled.

But football has changed. Arguably the most important clash in Saturday’s Champions League final between Juventus and Real Madrid will be the one between two fullbacks, Real Madrid's Marcelo and Juventus's Dani Alves.

The role of the fullback has been changing for some time. Even the term feels a little outmoded these days, dating from the very origins of modern football in the 1860s. Back then, sides fielded a goalkeeper, a fullback, two halfbacks and seven forwards. By the 1870s, 2-3-5 and the notion of two fullbacks had taken hold: they were the deepest-lying outfield players, players committed to defending.

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The terminology became confused as the W-M (3-2-2-3) took over following a change in the offside law in 1925, the center-half in the 2-3-5 dropping deep to become a third back, playing between the two fullbacks. But the term really became misleading after the 1958 World Cup, at which Brazil had introduced the world to the 4-2-4. By then, an additional midfielder had dropped into the defensive line and the two fullbacks played on the flanks of a back four, pushing into the space in front of them to join the midfield. In Portuguese and Spanish, they were known as “laterals”–their job was merely to be wide. But in English-speaking countries the old term remained, contradictory and misleading.

It was Ireland manager Jack Charlton, at the 1994 World Cup, who observed that when a 4-4-2 met a 4-4-2, the fullback was the most tactically important position on the field because he was the player with time and space in front of him. Gradually, he began to exploit that, pushing forward further and further. The shuttling wide midfielders of the 1990s and early 2000s–Steve Stone, Angelo Di Livio, Dennis Rommedahl–have been phased out, moved higher up the pitch or inside as creators, the flanks vacated for the marauding fullback. Certainly for top sides, when we speak of a back four, what we now really mean is a back two. In that context, the return of the back three and at least a little additional cover (although it is often accompanied by two holding midfielders) is an obvious reaction.

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The benefits of having a fullback who can get forward down one flank, can attack from deep, have long been recognized, but in the past the tendency was for a club to have an attacking fullback on one side balanced by a more defensive player on the other. It’s telling, with regard to Vialli’s point, that the great triumvirate of attacking fullbacks–Nilton Santos, Silvio Marzolini and Giacinto Facchetti–all played on the left.

But now it’s common for both fullbacks to attack. Dani Alves, a right back, perhaps began the process at Barcelona. He had his critics, those who protested he can’t defend, and they have a point. His positional sense is questionable and in certain moods he can dive in. But it didn’t matter. Barcelona usually had the ball, so his job was to support Lionel Messi, and Gerard Pique could cover. There were doubts about him at Juventus as well, but his impact on the semifinal, when he played as a wingback, was unquestionable, setting up three goals and scoring another, often coming inside almost as an auxiliary playmaker.

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Marcelo, similarly, has had his critics. Although he was by no means alone, his performance in Brazil’s 7-1 defeat to Germany in the 2014 World Cup semifinal was staggeringly ill-disciplined. But this season he has excelled. Perhaps he should have committed a foul as Barcelona broke to score the winner in the Clasico, but it was his cross that had brought Madrid the equalizer. In the Champions League quarterfinal, it was his slaloming run that led to Cristiano Ronaldo’s third goal, but it was his more disciplined performance in the semifinal against Atletico Madrid that really impressed.

Between them, the two Brazilians have set up 17 goals this season, and scored nine. They have won a combined 53 major titles. They have played against each other, in Clasicos, on 19 occasions, with Dani Alves leading 13 wins to three (and three draws). And on Saturday, the two best examples of the modern ideal of the fullback will go head-to-head for the biggest prize in the club game.