Ruy Lopez. Open Variation
The Open variation of Ruy Lopez begins with e4-pawn capture in the 5th move of the main line after 5.0-0:
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Nxe4
This reminds the Berlin Defense (3.Bb5 Nf6 with 4.0-0 Bxe4), but inclusion of the moves a7-a6 and Bb5-a4 adds new features to the position, since Black may at some point think of b7-b5 (in the main line of the Open variation it occurs after 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Nxe4 6.d4 - 6. ... b5 - a good resource for Black against White's pressure on e5). By the way, if Black plans the capture on e4, it is better for him to do it at once (5. ... Nxe4), than just one move later, after b7-b5 (in other cases White will of course have desire to defend e4) - 5.0-0 b5 6.Bb3 Nxe4?! - and White's position after 7.Re1 (for example, 7. ... Nc5 8.8.Bd5 Bb7 9.d4 Ne6 10.de) may become more pleasant than in case of the usual 5.0-0 Nxe4.
In some sources, the line 5.0-0 Nxe4 is called the Morphy Defense, in some sources the Morphy Defense is used for (3.Bb5 a6 4.Ba4) 4. ... Nf6, and in some sources the line 5.0-0 Nxe4 is called the Tarrasch Defense. In order to avoid all that mess, it looks logical to call the line 5.0-0 Nxe4 the Open Variation. Of course, the names of the lines or variations do not influence much the chess aspects of their positions .
The idea of Black's capture on e4 is creating space for his pieces (which is not the case, say, in many lines of the Closed variation).
After 5.0-0 Nxe4, the main line goes with 6.d4 (which is the most popular and is good of course), but there are some more ways for White:
6.Nc3 (Knorre variation) - White gets not much both after 6. ... Nxc3 and 6. ... Nf6. The position is equal, but Black's accurate play is required.
6.Qe2 (Tartakower variation) - the same, equal position after 6. ... Nc5 or 6. ... Ng5.
6.Bxc6 - 6. ... dc (not 6. ... bc? 7.Re1) 7.Re1 Nd6 8.Rxe5+ Be7, or 7.Qe2 Nf6 (or 7. ... Nc5, or 7. ... Ng5), or 7.Nxe5 Be7, etc.
6.d3 (as a rule, this pawn is needed much more on d4 in the Open variation) - with three good ways for Black - 6. ... Nc5, 6. ... Nf6, or 6. ... b5.
As mentioned, probably the best way for White after 5.0-0 Nxe4 is 6.d4. But there is one more way, even if not so ambitionus as the main line, but still with many interesting ideas (and not so harmless for Black) - 6.Re1. This looks in a way fresh (compared with the main line, especially 5.0-0 Nxe4 6.d4 b5 7.Bb3 d5), although in some sources the rook move is considered as "not very promising". I would not say so. While working on this line, I found it rather interesting (in my games, I play both 6.d4 and 6.Re1 fifty-fifty.
Here are some ideas on 6.Re1
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Nxe4 6.Re1
Firstly, this line is really not that harmless and requires Black's accurate play.
Black has to do something about the attacked knight - retreat it, defend it or, in his turn, attack the white piece - the bishop ... or try some kind of a gambit, say, 6. ... Nxf2 (?), or 6. ... Nxd2 (?) .
It's not a very good idea to defend the knight with a pawn, both 6. ... f5 (?) 7.d3 and 6. ... d5 (?) 7.Bxc6+ bc 8.d3 Nf6 (still worse for Black is 8. ... Nc5 9.Nxe5 (not 9.Rxe5+? Ne6) 9. ... Ne6 10.Qh5 or 9. ... Be6 10.Qh5, with unpleasant position for Black) 9.Nxe5 Be6 10.Nxc6 with possible 10. ... Qd6 11.Nd4, nothing good for Black.
Black may try to leave the attacked knight where it is, but attack the white bishop - 6. ... b5, which does not give so much for Black, both in case of 7.Bb3 Nc5 (7. ... d5?! 8.Nc3) 8.Bd5 Bb7 9.d4, and 7.Ne5.
As for the ways to retreat the attacked knight: 6. ... Ng5 (?) 7.Nxg5 (not 7.Nxe5) 7. ... Qxg5 8.d4, with good position for White, in connection with possible d4-d5 and Re1xe5+.
More or less playable for Black is 6. ... Nf6 7.Nxe5 Be7 8.d4 0-0 9.Nc3, or 6. ... Nd6 7.Nxe5 Be7 8.Qe2 (Black must be careful about the e7-bishop) 8. ... Nd4 (or 8. ... Nf5, but not 8. ... Nxe5? 9.Qxe5, with problems for e7) 9.Qg4 Ne6.
The best way for Black after 6.Re1 is 6. ... Nc5 (which is the most popular way, and is really good).
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Nxe4 6.Re1 Nc5
White has not a wide choice here. For example, in case of 7.Nc3 Ne6 Black equalizes easily, and in case of 7.Nxe5 Nxe5 8.Re5+ Be7 9.Bb3 0-0 (or 9. ... Nxb3) Black has nothing to worry about (7.Bb3 may lead to the same positions as 7.Nxe5).
Therefore, 7.Bxc6 is practically forced here.
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Nxe4 6.Re1 Nc5 7.Bxc6
As for Black's options, 7. ... bc is just slightly weaker (but quite playable) than the main move 7. ... dc.