Chess Opening Theory/1. d4/1...Nf6/2. c4/2...g6/3. Nc3/3...Bg7/4. e4/4...d6/5. f3

5. f3 is the Sämisch Variation, a well-known and popular line in the King's Indian. With this solid move white bolsters his centre. He remains noncommittal with regards to his king, with castling both queenside or kingside possible depending on what black does. This can be a tough nut to crack for black, who usually wants white to overextend when playing the King's Indian.

However, the Sämisch does have some drawbacks for white. Most obviously, kingside development is impeded because the king's knight cannot go to its usual square, f3. Secondly, the g1-a7 diagonal is open and on a direct line to white's king, should white castle short. The Sämisch has fallen somewhat out of favour at the highest levels of chess for the more natural-looking 5. Nf3, but it's still sometimes seen in grandmaster play. At the amateur level, this variation remains perfectly viable and gives both sides good chances.

1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. f3

Wikipedia has related information at King's Indian Defence, Sämisch Variation.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.