Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. Nc3/4...fxe4/5. Nxe4/5...Nf6/6. Nxf6/6...Qxf6/7. Qe2/7...Be7/8. Bxc6/8...dxc6/9. Nxe5/9...Bf5

What happens next in this position depends more on what Black is planning than on what White's next move is.

Scenario 1: White and Black both castle kingside, Black attacks the e5-knight with ...Bd6, White defends it with d4 and possibly f4 too, Black swaps it off and gets a light-square bind to compensate for White's extra passed pawn.

There are three routes by which Scenario 1 can come about: 10. O-O, 10. d4 and, surprisingly, 10. d3. White loses a tempo in the 10.d3 line by playing d2-d3-d4, but Black also loses a tempo by playing Ra8-e8-d8. It all makes sense - see below!

Scenario 2: White castles kingside, Black castles queenside, Black attacks the e5-knight with ...Rhe8 and ...Bd6, White counter-attacks with Qf2 and claims an advantage based on some devilish tactics along the f-file.

There are two ways of arriving at Scenario 2: 10. O-O and 10. d3. The 10.d4 line diverges if Black goes 10...O-O-O, because the pawn is immediately attacked and White has to take a move to defend it.

In fact, 10.d4 tends to provoke Black into playing 10...O-O-O to exploit the loose pawn - whereas against 10.d3 or 10.O-O, the main line of the 10...O-O-O variation scores extremely well for White. So to maximise the chances of ending up in the 10...O-O lines, White needs to play 10.O-O or 10.d3 here.

What's the difference between 10.O-O and 10.d3? This is a little complicated. 10.O-O means that White retains the option of playing d2-d4 in one move if Black castles kingside - and will usually exercise that option with 10...O-O 11.d4, after which follows 11...Bd6 12.f4 Bxe5 13.dxe5 Qg6 14.Rf2 Rd8. So surely if White goes 10.d3, Black can come out a tempo up by playing 11...Bd6 and 12...Rd8? Sadly for Black, the move ...Re8 turns out to be necessary to force White into playing d4. If Black doesn't bother threatening the e5-knight, White can instead play the manoeuvre Bc1-d2-c3, which would make the queen on f6 feel very uncomfortable.