Bishops are long range pieces that should be placed on long open diagonals. Unlike the knight, the bishop doesn't necessarily need an outpost in the centre of the board - it can exert influence on the centre all the way from the edge. The best position for a bishop is one where it commands an open diagonal, and cannot be attacked by enemy pawns or knights.
Fianchetto is an italian word meaning 'little flank'. It is a method of developing a bishop on b2 or g2 (for white) or b7 or g7 (for black) so that it sits on the longest diagonal from corner to corner, and fires right across the centre of the board. Here is an example - in this position, black has developed the dark squared bishop with a fianchetto on g7:
A bishop developed this way is called a fianchettoed bishop. From the flank, the bishop is able to control squares in the centre while a knight on the same square would be in the doldrums. The pawns around the bishop form a miniature fortress, shielding the bishop from attack and stopping an enemy knight from approaching too closely.
A fianchetto is a standard feature of many openings, but it isn't always the right choice. Fianchetto openings are often more difficult to handle than more straightforward development, and if the bishop moves away or is captured, it can leave behind holes in the pawn structure. If the king is castled behind a fianchettoed bishop, a common method of attacking is to exchange the bishop off, in order to exploit the holes left behind.
Two bishops working in tandem on adjacent diagonals are sometimes known as Horwitz bishops, after the 19th century master Bernhard Horwitz. With their ability to complement each other and control many squares in the opponent's camp, they can be incredibly powerful, as the following example shows:
As we have seen, bishops like clear, open diagonals to attack along. Because they only move on squares of one colour, bishops are vulnerable to being blocked by pieces on that square colour. The worst nightmare for a bishop is when it finds itself blocked by its own pawns, and the pawns are in turn blockaded by the enemy, dooming the bishop to perpetual imprisonment. A bishop that is blocked by its own pawns is known as a bad bishop. Here is an extreme example:
Here, white's bishop is stuck behind its own pawns, which are in turn blockaded by black. The bishop is next to useless - it can't ever do anything active and does little more than get in the way. Not only is the bishop bad, but because both bishop and pawns stand on the dark squares, the light squares are left unguarded. There is little white can do to stop the black pieces infiltrating on the light squares. Black is a pawn down, but he should eventually win the game because white's bad bishop is little more than a fancy pawn and no match for black's knight. This is a case where a bishop is worth a lot less than the usual 3 points!
So, it's clear we should try and avoid putting all our pawns on the same colour square as our bishop, but we start the game with two bishops on opposite coloured squares. We have to put our pawns on the same colour as one of them. If there are only a few pawns on the board this isn't usually a problem, but often it means you will have a good bishop and a bad bishop.
If you can't exchange your bad bishop for your opponent's good bishop, then exchanging it for a knight is almost as good. Failing that, a bad bishop can become a useful piece if you can station it outside your pawn structure where it can attack squares in the enemy camp. On the board below, white has placed the light squared bishop, which would otherwise be bad, on an active square outside the pawn structure:
An active bishop is always better than a passive prisoner, but it still suffers from being blocked by its own pawns - you should take care that this doesn't result in the bishop becoming trapped. A bad bishop might not be a good piece, but that still doesn't mean you can afford to lose it for nothing!
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