Fianchetto
Developing a bishop to the long diagonal via b2/g2 (or b7/g7), behind the knight's pawn moved one square.
A fianchetto is a particular way to develop a bishop. The knight’s pawn (g2 or b2 for White, g7 or b7 for Black) advances one square, and the bishop moves to the square the pawn vacated. The bishop then sits on a long diagonal — h1-a8 or h8-a1 — pointing across the entire board.
The fianchettoed bishop has both a strength and a vulnerability. Its strength is the diagonal: from g2, the bishop covers from h1 all the way to a8, and its influence is felt in every part of the board. Its vulnerability is the king’s shelter — fianchettoing weakens the long dark or light squares around the king, and if the fianchettoed bishop is ever traded off, those squares can become permanent holes.
The fianchetto is one of the central features of hypermodern openings. Réti’s Opening, the King’s Indian Defense, the Grünfeld, the Catalan, and the English Opening all feature kingside fianchetto setups. The Sicilian Dragon, the Pirc Defense, and the Modern Defense are the black-side cousins on the same theme. In each case, the bishop’s long-diagonal influence substitutes for direct central occupation.
The doubled fianchetto — both bishops fianchettoed, one on g2 and one on b2 — is the Hippopotamus or Larsen’s Opening territory. The setup has its admirers; its drawback is that pieces never reach the centre at all, and the king-side weakness behind the kingside fianchetto can become serious in a sharp middlegame.