The “Bishop pair” refers to a position where a player has both of the two Bishops. This means that they have both the light-square and dark-square bishop. Having the Bishop pair is considered to be an advantage, as each Bishop complements the other. Whereas individually the Bishops are considered to be worth about 3 Pawns each (or a little more), the Bishop pair confers some additional value on each Bishop, making the Bishop pair worth more than 6 Pawns.
A Bishop pair is considered a significant endgame advantage. In addition, the two bishops can checkmate an enemy King, even if there are no Pawns (whereas two Knights cannot checkmate). Two bishops are also particularly good at harassing Rooks in the endgame.
The Bishop pair is best in an open position, but is quite poor in a closed position. If the bishops are blocked in by pawn chains, then a Knight or two Knights in opposition may do better against the bishop pair.
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