The Bishop Pair, Weak Squares and Outposts

Two bishops beat bishop and knight for a geometric reason: each bishop sees only half the squares, and the two together see all 64. As pieces come off and the diagonals open, the pair works on both wings at once. A knight reaches both colours, but one at a time. The exception: in a closed position, with a hole to land on, the knight is the better piece.

Why two bishops beat bishop and knight

A lone bishop is half a piece: the light-squared one will never set foot on a dark square. With both, he has no safe square anywhere: a piece on a light square is hit by one bishop, a piece on a dark square by the other.

The advantage also grows on its own: every pawn trade opens diagonals, so a bishop worth little on move 15 is worth a lot on move 35 without having moved. Hence the rule for both sides: with the pair, open the game; against the pair, lock the position or trade one of them off.

• Open center: a long diagonal is what a bishop runs on.
• Pawns on both wings: the bishops work on both sides in the same move.
• Few pieces left: fewer blockers, more range.
• Enemy pawns fixed on one colour: one bishop attacks all of them at once.

Weak squares, holes and colour complexes

A square is weak when none of your pawns can defend it any more. Pawns do not move backwards: once the pawn that guarded it has advanced past it or been traded, the weakness is permanent.

• Weak square: no pawn of yours can attack or defend it again.
• Hole: a weak square inside your own camp, within reach of his pieces.
• Colour complex: several weak squares of one colour, from pushing every pawn to one colour or trading the bishop that covered the other.
• Outpost: a square in enemy territory, protected by a pawn of yours, that no pawn of his can reach.

Play ...g6 and ...Bg7 and the pawns on f7, g6 and h7 all sit on light squares, leaving the bishop on g7 as the only guard of the dark squares around the king. Trade it and f6, g7 and h6 belong to nobody. The only weak squares that matter are the ones he can occupy.

Good bishop, bad bishop

Good and bad bishops are about your own pawns, not his pieces. Pawns on your bishop's colour are walls it never crosses, and the bishop is bad. Pawns on the opposite colour and the two complete each other: one covers one colour, the other covers the rest.

Black's problem is not the bishop, it is the pawn chain parked on light squares. Three cures, in this order:

1. Trade the bad bishop. In the diagram, ...b6 and ...Ba6 offer it for White's good bishop. In the French Defense the classic route is ...Bd7-e8-h5.
2. Move the pawns to the other colour. A push like ...f6 or ...c5 gives the bishop air, at the cost of time.
3. Get the bishop out before the position closes. If the chain will settle on light squares, put your light-squared bishop outside it first.

When the good bishop is yours, the rule flips: retreat instead of trading, and fix his pawns on the wrong colour.

The outpost and how to build one

An outpost is a weak square with an address: it sits in enemy territory, a pawn of yours protects it, and no pawn of his can reach it. A knight parked there on the fifth or sixth rank usually outweighs any bishop. The Sveshnikov Sicilian is the most honest example, because Black creates the hole on purpose.

With 5...e5 Black kicks the knight off d4 and accepts that d5 will never be defended by a pawn again: the c-pawn left the board on move 4 and the e-pawn is already past d5. Black gets ...b5 and activity in return. White's whole plan becomes occupying d5, hence 9.Bxf6 gxf6: White gives up the bishop pair to remove the knight fighting for the square.

1. Find the square: somewhere his pawns can no longer reach. After ...e5, the black pawn gives up d5 and f5 forever.
2. Back it with a pawn: an outpost without pawn support is a visit, not a home. A pawn on e4 supports d5 and f5.
3. Remove the defenders: usually a knight. If that costs the bishop pair, the outpost has to be permanent.
4. Occupy it and stay: a knight that cannot be traded or chased away is an extra piece in that sector.

When the knight is better

Close the position and the ranking flips. Against a locked pawn chain the bishop bites on granite: it sees a long way and gets nowhere. The knight hops over the chain, changes colour every move and attacks squares the bishop cannot reach even in theory.

Outside locked positions, three things favour the knight: an outpost that will not disappear, blockading a passed pawn (it blocks and attacks at once) and all the pawns on one wing, where a bishop's extra range has nowhere to go.