Why does every club want a left-footed centre-back, and who is the best around?

Why nobody can argue with Benzema's Ballon d'Or win (1:24)

Shaka Hislop and Alejandro Moreno praise Karim Benzema after he won the Ballon d'Or. (1:24)

In the football world, the left-footed player has long been considered aesthetically superior -- more skilled, technically gifted and easier on the eye -- as opposed to those relying on their right, despite the evidence being overwhelmingly anecdotal or unsupported by science.

In bygone days this often resulted in the idea that the left-sided centre-back in a back four tended to be the more cultured and comfortable on the ball in a defensive pair, whereas the right-footed partner was often seen as the pure defender, tasked merely with defensive duties like sniffing out looming danger and taking care of the first duels.

As the art of defending has moved on this theory has not stood the test of time, but the value of fielding centre-backs with differing natural feet still persists. There are some obvious reasons why it works: when entering a one-vs.-one situation, a defender is understandably more comfortable in dealing with the opponent whose strong foot is closest to the byline, so any attempted cross into the box can be blocked with the defender's stronger foot and the positioning of their body movement feels more natural. Admittedly, today this is of less relevance with wide forwards, or inverted wingers, being two-footed or of the mind to cut inside and shoot rather than look for a cross like Arjen Robben used to.

Equally, as a general rule -- though there are endless exceptions -- defenders, who tend to be less agile than forwards, prefer to turn and position their bodies, often in a fraction of second, with the weight on their stronger foot. In a high-pace game broken down into an infinite number of fragmented moments, coming out of on top from such situations are likely to be crucial.

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In the age of the more-flexible defender -- both tactically and physically -- the preference for wanting a left-sided centre-back on the left is commonly found in the build-up phase or constructive side of the game. With central defenders being expected to start the attacks with the first pass and participating in the possessional aspect, the demand for precision is higher (and it's logically easier to execute with one's stronger foot), while one can also find better angles for a line-breaking ball into crowded, central areas.

Then there are technical details to consider such as a disguised pass, being at ease when successfully dribbling away from danger areas, or pin-pointing a crossfield switch ball -- which are all better executed on the stronger foot.

Furthermore, with many clubs fielding three centre-backs, the preference for a left-footer on the left (or vice-versa) of the trio assumes even more importance. With the wider centre-backs expected to advance with the ball out of the defence -- best done with clean, natural touches -- and even take up positions high up the pitch akin to an "inside full-back," the necessity for passing accuracy is even higher. When advancing into the attacking areas, the centre-back also occasionally ends up in crossing positions in the last third, rendering a spot-on delivery paramount.

Obviously, nowadays there are plenty of examples of centre-backs who are practically two-footed, meaning it matters less where they line up, as well as some world-class right-footed centre-backs who are used to playing to the left -- Chelsea's Kalidou Koulibaly and Liverpool's Virgil van Dijk being prime examples. Yet just an estimated 20% of players at the top end of the game are naturally left-footed (with an equal number being classed as two-footed), so the underrepresentation of top-quality left-footed centre-backs is also a matter of mathematics, simple economics, and supply and demand too. Logically this scarcity obviously makes an impact on the transfer fees.

Here are five of the best to be found playing in European football at the moment.

David Alaba, 29, Real Madrid

Arguably the top left-footed (though classed as two-footed by some) centre-back in the game, the Austria international's well-known flexibility -- he was originally a left-back at Bayern Munich before being switched to centre-back and central midfield -- makes him one of the most versatile too. Few other top-level defenders are also capable of performing as a winger with excellent crossing abilities as well as directing the play from a deep midfield role.

As a centre-back, Alaba takes maximum advantage of the skillset he employs from the more attacking and strategical roles: his precise long diagonal passes from the back often constitute a threat 60 yards up the pitch and his brilliant short passing (63 passes per game on average) and ball progression almost turns him into something akin to a playmaker.

Gabriel, 24, Arsenal

Gabriel's edge is certainly most on the physical side of the game than some of his predecessors in the position. By being tight in his marking, proactive and seeing danger early -- plus representing a constant threat from attacking set-pieces -- he's currently one of the most efficient defenders in his position at the top end of European football.

Although he's not too adventurous while travelling in possession, the Brazil international can control the ball too; he often finds options in the middle with line-break passes and the sensitivity of his left foot also sees him initiating attacks with a well-placed long ball.

Alessandro Bastoni, 23, Inter Milan

The emergence of Bastoni as one of the top ball-playing defenders in Serie A has seen him attract significant interest from abroad. As yet Inter have managed to hold on to the Italy international, who excels at picking progressive passes and participating in the ball circulation (1.1 key pass average per game this season) when stepping up from the left position in Inter's back three.

Smooth on the ball, Bastoni -- who particularly prospers as the left of three centre-backs -- also exhibits an understanding of the game rare for his age, which is visible in his early interceptions and reading of an attacker's next move. His aerial game, previously considered a weak spot, is also showing signs of improvement (62% success rate last season), making the 23-year-old one of the most complete left-sided centre-backs around.

Pau Torres, 25, Villarreal

While Spain are still trying to work out their ideal central defensive pairing in the post-Sergio Ramos era, the Villarreal man is making the left centre-back slot on their back four his own. Torres may not be the most aggressive or avid tackler, but he's exceptionally strong at picking the right positions, covering and reading defensive imbalances early.

In possession, he's equally smart -- habitually advancing through the first line of pressure with ease and picking out the right movement in midfield or further up the pitch. For a defender of his size, 6-foot-3, Torres is both deceptively quick over short distances and on the turn.

Josko Gvardiol, 20, RB Leipzig

At first glance, with his no-nonsense appearance and assertiveness in the duels, Gvardiol appears to be a traditional defensive "enforcer," but by looking deeper it quickly transpires that the 20-year-old already possesses most features of the complete centre-back.

Though his proactive style is arguably more suited to a three-man central defensive line -- in which he can take up high positions, roam forward with the ball and link up with the left-back and the creative midfielders -- Gvardiol is adapting to a slightly more settled role in a back four this season. Being particularly strong at spotting early attacking runs (and having the technical/passing quality to pick them out), the Croatia international typically sees a lot of the ball when his side are building from the back. One of the most coveted young centre-backs in European football, he is reportedly being monitored by practically every top club on the continent, including Man City and Chelsea.