Fianchetto

In chess, the fianchetto ( or ; "little flank") is a pattern of wherein a bishop is developed to the second rank of the adjacent b- or g-, the having been moved one or two squares forward.
The fianchetto is a staple of many "hypermodern" openings, whose philosophy is to delay direct occupation of the with the plan of undermining and destroying the opponent's central outpost. It also regularly occurs in Indian defences. The fianchetto is less common in Open Games (1.e4 e5), but the is sometimes fianchettoed by Black in the Ruy Lopez or by White in an uncommon variation of the Vienna Game.
One of the major benefits of the fianchetto is that it often allows the fianchettoed bishop to become more active. However, a fianchettoed position also presents some opportunities for the opponent: if the fianchettoed bishop can be , the squares the bishop was formerly protecting will become weak (see "") and can form the basis of an attack (particularly if the fianchetto was performed on the ). Exchanging the fianchettoed bishop should not be done lightly, therefore, especially if the enemy bishop of the same colour is still on the board.
Concept.
The adjacent diagram shows three different sorts of fianchetti (not as part of an actual game, but as separate examples that have been collapsed into a single chessboard). White's king bishop is in a regular fianchetto, with the knight pawn advanced one square and the bishop occupying the long diagonal. This is by far the most common type of fianchetto, seen in the Sicilian Dragon, Pirc Defence, Modern Defence, Modern Benoni, Grünfeld Defence and King's Indian Defence, among other openings.
Black's queen bishop is also fianchettoed, but the knight pawn has moved forward two squares, making this a "long" fianchetto. The b-pawn also controls the c4-square, which is often advantageous. If White plays the King's Indian Attack 1.Nf3 2.g3, Black may play a long queen's fianchetto to oppose White's bishop and make it more difficult for White to play a c4 . A long fianchetto on the kingside is more rarely played, because it weakens the pawn shield in front of the castled position and controls a less important square. Nevertheless, Grob's Attack 1.g4 and the Borg Defence ("Grob" backwards) 1.e4 g5 are sometimes played by players such as IM Michael Basman.
White's queen bishop has moved to a3 in what is sometimes called an "extended" fianchetto. Rather than control the long diagonal, it takes aim at Black's f8-square. If Black moves his e-pawn, White can play Bxf8, after which Black will have to waste on artificial castling after recapturing with his king. This tactic is often seen in the Evans Gambit and gives the Benko Gambit much of its bite. Black often plays ...Ba6 in the French Defence, and the Queen's Indian Defence if White plays g3 in order to fianchetto his own bishop (Aron Nimzowitsch's move against the classical main line).
Four fianchettoed bishops.
The game Rubinstein–Nimzowitsch, Marienbad 1925, had four fianchettoed bishops, two developed knights, and two on their home squares. In this position, Nimzowitsch humorously pointed out in "My System": "Each side castles now with a clear conscience, for not even the most hypermodern pair of masters can produce more than four fianchettoed Bishops!"
References.
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