The bishop pair

Each side starts the game with two bishops: one that only ever travels on light squares, and one that only ever travels on dark squares. A single bishop, no matter how well placed, can only ever influence half the board's squares. When you hold both of your bishops and your opponent has traded one (or both) of theirs away, you have 'the bishop pair.'

The pair is valuable because the two bishops cover for each other's blind spot: together they control every square on the board, light and dark alike, in a way that two knights or a knight-and-bishop pairing can't quite match. This tends to matter most in open positions with pawns spread across both flanks, where the bishops' long diagonals have room to work.

Giving up the bishop pair (trading one of your bishops for an enemy knight, for instance) isn't automatically bad -- sometimes the trade wins time, damages the opponent's pawns, or simply suits a closed position where bishops are cramped anyway. But all else being equal, the side with the pair usually has a small, durable structural plus that tends to grow as more pieces come off the board.

Unlike a tactic, the bishop pair isn't something that wins material on the spot -- it's a long-term asset, the kind of edge a player nurses for many moves rather than cashes in immediately.