Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Ba4/4...Nf6/5. O-O

White castles. White keeps the option to play c3 and d4 open by preparing to protect e4 with Re1. Once e4 is defended, White's threat to take e5 is renewed because Black's tactic with Qd4 would not longer win the pawn back.

Black must decide between several priorities: castling is one, which means choosing where to develop their kingside bishop ( 5...Be7 or 5...Bc5). At some point they'd like to defend their e5-pawn (5...b5 or 5...d6). Finally, they may be eyeing White's e4 pawn and thinking, isn't that hanging? (5...Nxe4)

Black needs to develop their f8 bishop before they may castle. It's two most credible squares are 5...Be7 and 5...Bc5.

5...Bc5, the Neo-Archangelsk variation, develops the bishop to the attacking g1-a7 diagonal and pins the f2 pawn. Bc5 means Black can defend the e-pawn with d6 without trapping the bishop behind the pawn chain. This line discourages 6. Re1?!, because after 6...Ng5 White finds themselves moving the rook back to f1 to defend their pawn. However, this makes White's other plan even better: White can gain tempo on the bishop with c3 and d4 and achieve the big centre they wanted.

For this reason 5...Be7 is the mainline.^[1] 5...Be7 is the Closed Spanish. Though it may look passive to develop the bishop behind the pawn chain, Be7 has several purposes. Firstly, it avoids losing tempo from c3 and d4. Secondly, it ensures White gets no tactics involving an e-file pin, not can they pin the knight with Bg5. Finally, by developing behind the pawn chain, the bishop can more readily help defend Black's kingside.

Once White plays Re1 (defending e4) or c3 (controlling d4), Black's Qd4 tactic, which up to now has deterred White from playing Bxc6 and Nxe5, will no longer work. Black has a couple of ways to defend it. Because e5 isn't under immediate threat, Black usually delays this decision until after they have developed their kingside bishop (see above).

5...b5 helps defend e5 by ending the threat of Bxc6 and is the most flexible approach. After 6. Bb6, Black can play Be7 and d6 anyway, transposing into the Closed Spanish, but they have other choices. 5...b5 made room for Black to fianchetto their bishop to b7, the Archangelsk variation, where it (along with ...Bc5 if Black chooses) pressures White's kingside. 5...b5 is the main sideline.^[2]

In many lines, Black plays ...d6 to defend their e5 pawn, which ends the threat of Nxe5 and liberates Black's c6 knight from guard duty (it may instead chase down White's Spanish bishop). Black can choose to play it now, 5...d6, the Deferred Steinitz. 5...d6 closes in the kingside bishop, but as we have seen it is happy to develop to Be7 (transposing into the Averbakh variation of the Closed Spanish) or, more commonly, Black plays g6 and fianchettos it. Black does pin their own knight, but they can now play Bd7 to unpin it or Bg5 to pin White's knight.

Like the Modern Steinitz (4...d6), the Deferred Steinitz is an improvement over the Old Steinitz (3...d6 before playing a6) because Black now has the option of b5. If White tries 6. d4?, the usual approach to the Old Steinitz, then 6...b5 7. Bb3 Nxd4 8. Nxd4 exd4 wins a pawn, because 9. Qxd4? runs into 9...c5 10. Qc3 c4 trapping White's bishop―this is called the Noah's Ark trap, because it's such an old trap it might as well be in the Bible.

5...Nxe4 is the Open Spanish. For the moment, White has left the e4 pawn undefended. Black can take it this turn before White gets in the move Re1, though as usual when a player grabs a pawn in the Ruy Lopez, they find they can't hold onto it.

White can get Black's e-pawn (the usual continuation is 6. d4 b5 7. Bb3 d5 8. dxe5) and this leads to open positions with a very different flavour from the "torturous" Closed Spanish games. The Open Spanish is the third line, after the Closed Spanish and 5...b5.

5...d5? is the irrational Central countergambit where Black attacks in the centre. Black offers White their choice of central pawns to capture... but capturing either is quite good for White. This is even worse than the 3...d5?, the so-called Spanish countergambit, because White has safely castled and there is even less opportunity for Black to find counterplay.

5...g6, the Brix variation^[3], prepares to fianchetto the kingside bishop instead, like a deferred-deferred Smyslov.

For explanation of theory tables, see theory table and for notation, see algebraic notation.

1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O

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Wikipedia has related information at Ruy Lopez

• Modern Chess Openings 15th ed, 2008. Nick de Firmian. Random House, New York. ISBN 0-8129-3682-5.

• Nunn's Chess Openings. 1999. John Nunn (Editor), Graham Burgess, John Emms, Joe Gallagher. ISBN 1-8574-4221-0.