By far the most important category of the semi-closed openings are the Indian systems, which begin 1.d4 Nf6.
As these defenses have much in common and have a great deal more theory than all the remaining semi-closed openings put together, they are treated in a separate article; see Indian defense for details.
As well, 1...e6 is sometimes used by players wishing to play the Dutch Defense (1.d4 f5) without allowing White the option of 2.e4!
The Dutch, an aggressive defense adopted for a time by World Champions Alekhine and Botvinnik, and played by both Botvinnik and challenger David Bronstein in their 1951 world championship match, is still played occasionally at the top level by Short and others.
The Wade Defence, a slightly offbeat but fully playable line, arises after 1...d6 2.Nf3 Bg4.