 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.The third most common response to 1.d4 (after 1...Nf6 and 1...d5) is 1...e6.