Conventional wisdom suggests that a three-man defence is particularly effective against two strikers: there are two players to mark with a spare man floating just behind. Traditionally, problems are encountered against a lone striker, which leaves a surplus of defenders, meaning a side is secure at the back but undermanned elsewhere -- particularly against a quick front three. That was Everton's problem when deploying a 3-4-3 against Chelsea's 3-4-3 on Nov. 5: their defenders simply couldn't cope with the sheer speed of Pedro, Hazard and Diego Costa and were caught out on the counterattack quickly after conceding possession. They went on to lose 5-0.

That's a simplification, however. In an era when systems are more fluid and more defenders are capable of stepping forward to act as supplementary midfielders, perhaps the nature of the opposition system has become less relevant.

Tottenham were quietly impressive with a modified 3-4-1-2 against Arsenal's 4-2-3-1 because the centre-backs are all comfortable in possession and happy holding a high line. Spurs probably held the ball for too long in defence, playing a succession of square passes without really penetrating Arsenal, but their approach at turnovers worked well. Mauricio Pochettino's side would quickly move the ball to find two strikers playing up against two Arsenal centre-backs, who missed the spare man they've become accustomed to as so many teams play a lone striker away at the Emirates. Spurs' system meant they could play two up front without being overrun in the centre of midfield.

Perhaps most intriguing is Guardiola's experiments at Manchester City. Sometimes he has played a system that appears something like a 3-2-2-3, most notably in the 1-1 home draw with Everton, when City were extraordinarily dominant. Even when City have started with a four-man defence, they've often subtly shifted into a three, with holding midfielder Fernandinho dropping back, but occasionally -- such as away at Barcelona -- with a full-back drifting infield to become an extra midfielder.