Bishop pair
Both bishops of the same colour on the board — usually a long-term advantage, especially in open positions.
The bishop pair is one of the most studied small advantages in chess. Two bishops cover every square on the board between them — one travels on light squares, the other on dark. Together they exert pressure no other piece combination can match.
The pair’s strength varies with the position. In open positions, with few pawns to block their diagonals, two bishops are noticeably superior to two knights or to a bishop-and-knight combination. In closed positions, the pair’s value drops sharply: long diagonals are blocked, and knights — which can jump over pawns — become more useful.
The bishop pair’s value is often quoted as roughly half a pawn in an open middlegame, though this is a rough average. In endgames the pair’s value grows: with fewer pieces on the board, the bishops can dominate squares of both colours and produce zugzwang for the defender. In opposite-colored bishop endings, the pair is irrelevant because there is no pair — one bishop each.
In opening theory, surrendering the bishop pair is a recurring decision. The Nimzo-Indian’s main lines see Black trade the dark-squared bishop for a knight on c3, accepting the pair concession in exchange for damaged White pawns. The trade is theoretically respected because the structural gain compensates; whether it does in any specific position is one of the deepest opening-theoretical questions.