Castling notation — what it means in chess
O-O for kingside, O-O-O for queenside.
O-O for kingside, O-O-O for queenside.
[[board startfen="rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1" caption="Three developing moves each, and then the move written O-O: king two squares toward the rook, rook over the king. One notation, two pieces, the only move in chess that moves both." pgn="e4 e5 Nf3 Nc6 Bc4 Bc5 O-O" notes="The starting position — six developing moves from here, the scoresheet will need a symbol that names no piece and no square. A standard Italian opening begins — watch the notation when the king finds shelter. Black mirrors. Develop toward the center. Mirrored again. The bishop takes its diagonal. And again — both sides one move from castling. O-O: kingside castling. The king travels two squares to g1 and the rook crosses to f1, recorded as a single move with its own special symbol — the only notation in chess that names no piece and no square."]]
Castling gets its own notation because it is chess's only two-piece move: O-O for kingside (short) castling, O-O-O for queenside (long). The symbols count the rook's crossing — two squares short, three squares long — and are written with capital letter O's by convention, though the digit form 0-0 appears in older books and both are universally understood. No piece letter, no destination square: the notation names the maneuver itself.
Reading it back onto the board: kingside, the king goes from its home square two squares toward the h-rook (to g1 or g8) and the rook lands beside it on f1 or f8; queenside, the king travels two squares toward the a-rook (to c1 or c8) and the rook crosses to d1 or d8. The king moves two squares in both directions — the "long" in long castling belongs to the rook.
The Endgame Society glossary defines more than 240 chess terms — rules, notation, tactics, strategy, pawn structures, endgames, and openings — in language a beginner can act on rather than dictionary shorthand.