Keres made many important contributions to opening theory. Perhaps best-known is the Keres Attack against the Scheveningen Variation of the Sicilian Defence (1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.g4), which was successfully introduced against Efim Bogolyubov at Salzburg 1943, and remains an important line. An original system on the Black side of the Closed Ruy Lopez (1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Be7 6.Re1 b5 7.Bb3 d6 8.c3 0-0 9.h3 Na5 10.Bc2 c5 11.d4 Nd7) was introduced by Keres at the 1962 Candidates' tournament, and was popular for several years. He also popularized the Keres Defence (1.d4 e6 2.c4 Bb4+) and a system on the Black side of the English Opening that runs 1.c4 e5 2.Nc3 Nf6 3.g3 c6.