If Black is not careful, there is the risk of running unprepared into a highly theoretical opening, e.g. after 1.Nf3 c5 White can play 2.e4 leading to the mainline Sicilian Defense.

Other common transpositions are to various lines of the Queen's Gambit Declined (after e.g. 1.Nf3 d5 2.d4 Nf6 3.c4) or the Catalan Opening (after e.g. 1.Nf3 Nf6 2.g3 d5 3.Bg2 e6 4.0-0 Be7 5.c4).

[6] The main independent lines which usually start with 1.Nf3 are the Réti Opening (1.Nf3 d5 2.c4) and the King's Indian Attack (where White plays 1.Nf3, 2.g3, 3.Bg2, 4.e4, and 5.d3, though not always in that order).

The Black responses which are given one or more chapters in the Encyclopedia of Chess Openings are given below, ranked in order of popularity.

1.Na3: Durkin 1.a3: Anderssen 1.b3: Nimzowitsch-Larsen 1.Nc3: Dunst 1.c3: Saragossa 1.d3: Mieses 1.e3: Van 't Kruijs 1.Nf3: Zukertort 1.f3: Barnes 1.g3: Benko 1.Nh3: Amar 1.h3: Clemenz