Castling
DEFINITION
A special king-and-rook move — the only move in chess that moves two pieces at once — that improves king safety by tucking the king behind its pawns and connects the rooks for the middlegame. Notated O-O (kingside) or O-O-O (queenside).
Why It Matters
Castling by move 10 is a widely used benchmark for completing king safety before the middlegame begins. CircleChess's analysis of 39,862 beginner games found only 47% castle by move 10, and 33% never castle at all in games lasting 10+ moves — making it, alongside piece development, one of the two opening habits most worth deliberate coaching.