Stanley Variation, Reversed Spanish

With 3...Bb4 Black adopts the Ruy Lopez idea with colors reversed. The bishop pins the c3-knight, which is the main defender of e4, so the latent threat is ...Bxc3 followed by ...Nxe4, just as White in the Spanish eyes the knight guarding e5. Unlike the fork trick 3...Nxe4, Black keeps material and development simple, and the play tends to be roughly level, though both sides must handle the tension on e4 with care.

The most popular answer is 4. d3, which shores up e4 and frees White from worrying about the exchange on c3. 4. Nf3 is also common, while 4. Nge2 prepares to recapture on c3 with a knight and avoid doubled pawns. 4. Nge2 gives White the best results of the main options in the Lichess sample. The engine also favors 4. a3, putting the question to the bishop at once. White can still play the sharp 4. f4, a belated Vienna Gambit, when 4...Nxe4 5. Qh5 O-O produces a messy position considered balanced. Across the sample White scores somewhat better than Black, and draws are uncommon.