edition of his chess treatise, Johann Allgaier introduced the 0-0 notation. He differentiated between 0-0r (right) and 0-0l (left). The 0-0-0 notation for queenside castling was introduced in 1837 by Aaron Alexandre. The practice was adopted in the first edition (1843) of the influential Handbuch des Schachspiels and soon became standard. In English descriptive notation, the word "Castles" was originally spelled out, adding "K's R" or "Q's R" if disambiguation was necessary; eventually, the 0-0 and 0-0-0 notation was borrowed from the algebraic system. Strategic and tactical concepts Strategy Castling is generally an important goal in the opening: it moves the king to safety away from the of the board, and it moves the rook to a more active position on a center file. The choice regarding to which side one castles often hinges on an assessment of the trade-off between king safety and activity of the rook. Kingside castling is generally slightly safer because the king ends up closer to the edge of the board and can usually defend all of the pawns on the castled side. In queenside castling, the king is placed closer to the center and does not defend the pawn on the a-; for these reasons, the king is often subsequently moved to the b-file. In addition, queenside castling is initially obstructed by more pieces than kingside castling, thus taking longer to set up than kingside castling. On the other hand, queenside castling places the rook more efficiently on the central d-file, where it is often immediately active; meanwhile, with kingside castling, a tempo may be required to move the rook to a more effective square. One may forgo castling for various reasons. In positions where one's opponent cannot organize an attack on one's centralized king, castling may be unnecessary or even detrimental. In addition, a rook can be more active near the edges of the board than in the center in certain situations, such as if it is able to fight for control of an open or semi-open file. Kingside castling occurs more frequently than queenside castling. It is common for both players to castle kingside, somewhat uncommon for one player to castle kingside and the other queenside, and rare for both players to castle queenside. If one player castles kingside and the other queenside, it is called opposite castling or opposite-side castling. Castling on opposite sides usually results in a fierce fight, as