The tenth edition of Modern Chess Openings (1965) grouped the Pirc and Robatsch together as the "Pirc–Robatsch Defense".

The Modern Defense, Averbakh System (ECO A42) can be reached by the lines: Possible moves for Black at this point include 4...Nf6, 4...Nc6, 4...e5, and 4...Nd7.

The flexibility and toughness of the Modern Defense have provoked some very aggressive responses by White, including the crudely named Monkey's Bum, a typical sequence being 1.e4 g6 2.Bc4 Bg7 3.Qf3.

(A more refined version is the Monkey's Bum Deferred, where White plays Bc4 and Qf3 only after developing the queen's knight.)

If White plays 3.Nc3 instead of 3.e5, Black can transpose to the Pirc Defence with 3...d6 or continue in unconventional fashion with 3...d5!?)