Perhaps the most fundamental of all football debates over the past two decades has been that over the number of players deployed in central midfield. In the early 1990s, playing with two forwards was seen as the natural state, and to withdraw one of them was regarded -- in England at least -- as an act of unspeakable negativity, permissible only for sides playing away in European competition (and even then only against tough opponents and in difficult conditions). More recently, the prevalence of 4-3-3 and 4-2-3-1 -- only at Euro 2008 did 4-2-3-1 finally become a more common formation at a major tournament than 4-4-2 -- has resolved football to the idea of the lone striker supported by runners from deep. This season, though, seems to have seen a reversion: suddenly and unexpectedly, striker partnerships are back.

Manchester City is perhaps the most eye-catching example. Even after the disappointment of Monday's 1-0 home defeat to Chelsea , it remains on course to post an end-of-season goals tally not matched since the early 1930s, and that is in large part due to Alvaro Negredo and Sergio Aguero (with Edin Dzeko to slot in as required). Paris Saint-Germain , similarly, is sauntering to the Ligue 1 title with a front two of Edinson Cavani and Zaltan Ibrahimovic. Juventus is streets clear in Serie A with Fernando Llorente and Carlos Tevez up front, while Atletico Madrid is leading Spain with a pairing of Diego Costa and David Villa.

Playing with two forwards remains a risky policy, as City's defeat to Chelsea showed: with an extra man in the center, Chelsea was able to dominate the back of midfield, erecting a barrier of David Luiz and Nemanja Matic, through which City found it extremely difficult to break. More generally, teams playing 4-4-2 always risk being overmanned in the center, which came make it hard for them to dominate possession.

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But what this season has shown is that the risk is manageable, and that there are significant advantages to playing with two forwards. At the highest level, the modern partnership differs profoundly from traditional strike pairings. Back then, the duos fell largely into two categories; there was the target-man, quick-man school, as exemplified by Mark Hateley and Ally McCoist for Rangers; Niall Quinn and Kevin Phillips for Sunderland ; Jan Koller and Milan Baros for the Czech Republic. There was also the creator, finisher school; as exemplified by Kenny Dalglish and Ian Rush for Liverpool ; Peter Beardsley and Gary Lineker for England; Roberto Mancini and Gianluca Vialli for Sampdoria .

With the shift to lone striker systems, forwards changed. In effect, they became strike-pairings in and of themselves, having to be both target-man and quick-man, or creator and finisher. The modern forward tends to be far more multi-faceted than his predecessors, used to dropping deep or pulling wide, or holding the ball up, as well as sniffing around the box in the search for chances. Pair two such players and the result is a highly fluid, highly mobile and highly unpredictable unit that is extremely difficult to combat.

Take Negredo and Aguero, for instance: although Negredo is bigger and a more natural leader of the line, he regularly ends up playing deeper than Aguero. In the past, the bigger centerback would usually take the bigger forward, and the quicker one the more nimble opponent. Now, the pairings are too fluid for such a simple division of labor.

Central defenders also face the difficulty that they have largely forgotten how to defend against pairs. Against a single forward, one defender goes to mark or challenge for the ball and the other drops off as cover. It's a relatively simple skill to master and perhaps explains why so few goals are scored these days from forwards running onto through-balls and meeting the goalkeeper one-on-one. Play two strikers and that safety net has gone.

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The risk is the shortfall in midfield, and what's fascinating is that City, Atletico, PSG and Juve have all found different ways to combat it. City effectively adds half a body to the center by the way David Silva plays narrow on the left, tucking in to support Fernandinho and Yaya Toure. Atletico at times plays almost a 4-2-2-2 rather than a 4-4-2, with Koke and Arda Turan both operating narrow. PSG often blurs the edges of the pairing and play what is effectively a 4-3-3, with Cavani drifting right and a more natural wide player on the left, with a solid core of three central midfielders -- usually Blaise Matuidi, Thiago Motta and Marco Verratti. And Juve follows the Italian vogue for a 3-5-2, the general lack of width in Serie A meaning it can turn fullbacks into wingers and so still operate with three central midfielders.

It was the great Russian coach of Dynamo Kyiv, Viktor Maslov, who noted that football was like an aeroplane, always becoming more streamlined. For over four decades, Maslov's theory holds true. This season suggests that the trend perhaps is being reversed.