In the Modern Defense, also known as the Robatsch Defense, after Karl Robatsch, Black allows White to occupy the center with pawns on d4 and e4. Black's hope is to attack and undermine White's "ideal" position without directly attempting to occupy the center himself. A typical move order for the Modern Defense is 1.e4 g6. Main line is 2.d4 Bg7. The Modern Defense has been most notably used by British GMs Davies and McNab.
The Modern Defense is quite closely related to the Pirc Defense, to the extent that the "Modern" is often seen as a description of some variants of the Pirc; the primary difference between the two openings is that in the Modern Black delays developing his knight to f6. Transpositional possibilities between the two defences are rife. The flexibility and toughness of the Modern Defense has provoked some very aggressive responses by White, including the attack crudely named the Monkey's Bum (a typical sequence being 1.e4 g6 2.Bc4 Bg7 3.Qf3), a more refined version of which is the Monkey's Bum Deferred, where White only plays Bc4 and Qf3 after developing his queen's knight.
The Encyclopedia of Chess Openings classifies the Modern Defense as B06. Codes B07 to B09 are assigned to the Pirc.
Other unusual openings can be reached after 1.e4 g6. The Hippopotamus Defense is one such system. Another is the Norwegian Defense, also known as the North Sea Defense, which begins 1.e4 g6 2.d4 Nf6 3.e5 Nh5. If White plays 4.g4, Black retreats the knight with 4...Ng7. On 4.Be2, Black can retreat the knight or gambit a pawn with 4...d6!? If White plays 3.Nc3 instead of 3.e5, Black can transpose to the Pirc Defense with 3...d6 or continue in unconventional fashion with 3...d5!?.
The delaying of Nf6 allows blockading the Bg7 with 3.c3.
Main Lines of the Modern Defense are 3.Nc3 d6 4.f4 c6 5.Nf3 Bg4 and 3.c4 d6 4.Nc3 Nc6 5.Be3 e5 6.d5 Ne7. 7.g4 will be answered by f5 8.gxf5 gxf5 9.Qh5+ Ng6 10.exf5 Qh4 11.Qxh4 Nxh4 12.Nb5 Kd8