A CTN record contains six fields. The separator between fields is a space. The fields are:
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Piece placement with white starting on the left. Eeach piece is identified by a single letter; i.e., P = pawn, N = knight, B = bishop, R = rook, Q = queen, K = king. White pieces are denoted using uppercase letters and black pieces use lowercase. Empty spaces are individually noted using periods.
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Active color. "w" means White moves next, "b" means Black moves next.
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Castling availability. If neither side can castle, this is "-". Otherwise, this has one or two letters: "K" (White can castle), and/or "k" (Black can castle). A move that temporarily prevents castling does not negate this notation.
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En passant target square index. If there's no en passant target square, this is "-". If a pawn has just made a two-square move, this is the position "behind" the pawn. This is recorded regardless of whether there is a pawn in position to make an en passant capture.
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Halfmove clock: The number of halfmoves since the last capture or pawn advance, used for the fifty-move rule.
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Fullmove number: The number of the full move. It starts at 1, and is incremented after Black's move.