Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Ba4/4...b5

Black drives White's bishop away. Black ends White's pressure on their knight, therefore ends the possibility of Bxc6 and makes their e5 pawn more secure.

While ...b5 is a common move in Ruy Lopez at some point, it is unusual to play it immediately on move four, because Black's e5 pawn was still defended tactically (Bxc6 dxc6 Nxe5 Qd4! forks White's knight and e4 pawn).

5. Bb3 is the only move. White preserves their bishop and gets the free tempo to move it onto the dangerous a2-g8 long diagonal, eying Black's vulnerable f7 pawn.

4...b5 made early appearances in the mid-19th century. Charles Henry Stanley (1819―1901) played it with the against Eugène Rousseau (1805―1870) in New Orleans in 1845, with the continuation 5. Bb3 Nf6.^[1] The Graz variation, with 5. Bb3 Bc5, was also common, as played against Morphy in an 1859 consultation game.^[2]

4...b5 is named after Horatio Caro (1862―1920), the British-German chess master known for the Caro-Kann defence, who played it from the 1880s. He introduced the continuation 5. Bb3 Bb7 and would play by fianchettoing both bishops.^[3][4][5]

The modern mainline continuation, 5. Bb3 Na5, was played by the likes of Mark Taimanov and others,^[6] but received the Norwegian soubriquet for Svein Johannessen (1937―2007), a Norwegian IM who adopted the line.^[7]

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 b5

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Wikipedia has related information at Ruy Lopez

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.