Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Ba4/4...Nf6/5. O-O/5...Be7/6. Re1/6...b5/7. Bb3/7...O-O/8. c3/8...d5/9. exd5/9...Nxd5/10. Nxe5/10...Nxe5/11. Rxe5/11...c6/12. d4/12...Bd6/13. Re1/13...Qh4/14. g3/14...Qh3/15. Re4/15...g5

The 90's Variation: 16. Qf3
This was the choice of Dutch super-GMs Jan Timman and Gert Timmerman in the 1990's, but it's been under a cloud ever since Michael Adams with the Black pieces knocked off Veselin Topalov in a Sarajevo tournament in 2000. The automatic response is after which 17.Re1 would be an abject retreat, so the uncompromising follow-ups for White involve sacrificing the exchange for a second Black pawn, with either 17.Bxd5 cxd5 18.Re3 Be4 19.Rxe4, or (Topalov's choice): when both Topalov's liquidating 20.Bxd5 and the retreat 20.Bd3 as in Timmerman-Slavchev 1991 would leave Black with a powerful threat of ...f4 activating the f8-rook in situ and ruining White's kingside pawns for the endgame. So 16. Qf3 was abandoned (no 2550+ player has tried it since 2001), in favour of...
 * 16...Bf5,
 * 17.Bc2 Bxe4 18.Bxe4 Qe6 (to defend d5 again) 19.Bxg5 f5,

The Modern Variation: 16. Qf1
This has been the preference of an all-star cast of current Ruy Lopez players: Anand, Kramnik, Shirov, Svidler, So, Vachier-Lagrave, Bacrot and Short to name a few. White evicts the Black queen from h3, and isn't too concerned about ...Qxf1+ in response which would leave Black a pawn down with no attack. Instead, keeps some threats on the board. Now that the c1-bishop doesn't really have a good square, White might as well develop the knight instead with and now Black has two ways of gaining time at the expense of the e4-rook.
 * 16...Qh5
 * 17.Nd2
 * 17...Bf5 can be met by 18.f3! when
 * 18...Bxe4? 19.fxe4 brings the b3-bishop back to life to great effect; after something like 19...Ne3 20.Qf3 Qxf3 21.Nxf3 White has a pawn, the bishop pair and enormous positional compensation for the exchange.
 * 18...Nf6 19.Qg2 (threatening a fork on g4 by guarding the h2 square) 19...Qg6 20.Re3 and White has good prospects of surviving the attack without having to sacrifice the exchange. 20...Rae8 21.Ne4 Nxe4 22.g4! Ng3 23.hxg3 and White has built a wall along the third rank.
 * 17...f5 and now the intermezzo 18.Bd1 scores excellently for White: 18...Qh6 19.Re1 f4 20.Ne4 (aiming for c5) 20...Bc7 and now 21.Bd2 has been the most common move with continuing complications.