[4] A popular use of the Ruy Lopez opening did not develop, however, until the mid-19th century, when the Finnish and Russian[5] theoretician Carl Jaenisch "rediscovered" its potential.

[8] White's 3.Bb5 is still a good move, however; it develops a piece, prepares castling, and sets up a potential pin against Black's king.

The Morphy Defence thus "puts the question"[11] to the white bishop, a traditional usage which Larry Evans attributed to Aron Nimzowitsch.

[12] The main point of 3...a6 is that after the common retreat 4.Ba4, Black will have the possibility of breaking a future pin on the queen knight by playing ...b5.

An influential chess player at that time, Wilhelm Steinitz, did not approve of the move, however; in 1889, he wrote, "on principle this ought to be disadvantageous as it drives the bishop where it wants to go".

Max Euwe gives the pure pawn ending in this position (with all pieces except kings removed) as a win for White.

[15] Black gains good compensation in the form of the bishop pair, however, and the variation is not considered White's most ambitious, though former world champions Emanuel Lasker and Bobby Fischer employed it with success.

In the late 19th and early 20th centuries, Emanuel Lasker had great success with 5.d4 exd4 6.Qxd4 Qxd4 7.Nxd4, most notably his famous win against José Raúl Capablanca in the St. Petersburg 1914 chess tournament.

Jon Jacobs wrote in the July 2005 Chess Life (p. 21): "A database search (limited to games longer than 20 moves, both players FIDE 2300+) reveals the position after 7.Nxd4 was reached 20 times from 1985–2002.

Alexander Alekhine played this for Black in the early portion of his career; despite his advocacy, it never achieved great popularity, and even he eventually came to consider it dubious.

Robert Wormald (1834–1876) wrote openings manuals and completed Staunton's last book, analysing the line in Chess World, 1867.

White has several options, including attempting to build an ideal pawn centre with c3 and d4, defending the e-pawn with Re1 or simply developing.

[26][27] In the Open Defence, 3...a6 4.Ba4 Nf6 5.0-0 Nxe4, Black tries to make use of the time White will take to regain the pawn to gain a foothold in the centre, with play usually continuing 6.d4 b5 7.Bb3 d5 8.dxe5 Be6 (but not 7...exd4?

Here 8.Nxe5, once adopted by Fischer, is much less often seen, and Black should equalise after the accurate 8...Nxe5 9.dxe5 c6, which avoids prematurely committing the light-squared bishop and solidly defends d5, often a problem in the Open.

After 9.c3 Bc5 10.Nbd2 0-0 11.Bc2, Black must meet the attack on e4, with the following possibilities from which to choose: 11...f5, 11...Bf5, both of which aim to maintain the strongpoint on e4, or the forcing line 11...Nxf2, introduced by the English amateur Vernon Dilworth.

[30] An analysis of the line had just been published in a Russian chess magazine, and Smyslov was able to follow it to quickly obtain a winning position.

Paul Keres played this line against Max Euwe and Samuel Reshevsky at the World Chess Championship tournament 1948.

This variation played a decisive role in a later World Championship match, Kasparov–Anand 1995, when Anand was unable to successfully defend as Black.

Play may also in some rare cases transpose to a traditional closed Spanish (with 7...d6) after something like 6.d3 d6 7.c3 0-0 8.Re1 b5 9.Bc2 Bb7 10.Nbd2 Re8 11.h3 Bf8 12.d4, reaching a reasonably well-trodden position in the Zaitsev system, though both players may deviate at many points in this line.

In the case of Flohr–Zaitsev-type setups, White may quickly push d3–d4 without h3 and lose a tempo compared to the traditional Zaitsev, but the move h3 is what would be lost, which is not so relevant with the bishop already committed to b7, so the only thing h3 provides is luft (an idea shared with the Pilnik).

Black generally goes all-in with a massive kingside attack, which has been analysed to great depth (sometimes beyond move 30) with no definite conclusion as to the Marshall's soundness.

and the Marshall Attack was adopted by top players including Boris Spassky, John Nunn, and more recently Michael Adams.

White aims to play d4 followed by Nbd2–f1–g3, which would firmly support e4 with the bishops on open diagonals and both knights threatening Black's kingside.

The Chigorin Variation has declined in popularity because Black must spend some time bringing his offside knight on a5 back into the game.

The main line runs 10.d4 Bxb3 11.axb3 (11.Qxb3 is another option) exd4 12.cxd4 d5 13.e5 Ne4 14.Nc3 f5 15.exf6 Bxf6 16.Nxe4 dxe4 17.Rxe4 Qd5 18.Rg4, when it has been shown that White's extra pawn is more valuable than Black's more active and harmonised pieces.

The loss of a tempo with 9...h6 gives White enough time to complete the Nbd2–f1–g3 manoeuver, and the pawn move can also weaken Black's kingside.

Kasparov played the Smyslov Variation in a loss to the Deep Blue chess computer in Game 2 of their 1997 Man vs. Machine match.

[63][64][65][66] Despite its drawish tendency, the Berlin endgame remains the sternest test of the entire variation beginning with 3...Nf6, and a lot of theory has evolved.

The Schliemann Defence or Schliemann–Jaenisch Gambit (ECO C63), 3...f5,[21] is a sharp line in which Black plays for a kingside attack, frequently sacrificing one or two pawns.

This defence was published in 1843 in Paul Rudolf von Bilguer's Handbuch des Schachspiels and explored by Henry Bird in the late 19th century.