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The Troitzky (or Troitsky) line is a key motif in chess endgame theory in the rare and practically unimportant (but theoretically interesting) ending of two knights versus a pawn. The endgame was analyzed by A. A. Troitzky.

Whilst two knights cannot force checkmate (with the help of their king) against a lone king, ironically a decrease in material advantage allowing the defending king to have a pawn can actually cause his demise. This is due to the fact that a common technique in this endgame is that of reducing the defending king to a position that would be a stalemate except for an available pawn move, and allowing the pawn to move can allow the attacking knights to move in for the kill. For the position with White on the attack, Troitsky established that if a black pawn is blockaded (by one of White's knights) on a square no further forward than the line a4-b6-c5-d4-e4-f5-g6-h4, then White can win the resulting endgame (and similarly in reverse for Black), no matter where the other pieces are placed. However, the checkmate procedure is difficult and long. In fact, it can require up to 115 moves by White, so in competition usually a draw by the fifty move rule will occur first. Therefore the ending is more of theoretical than practical interest. If Black's pawn is past the Troitsky line, there are zones such that if the black king is in one, white still has a theoretical win; otherwise the position is a draw.

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Here is an example of how having the pawn makes things worse for Black (here Black's pawn is past the Troitsky line), by making black have a move available instead of being statemated. 1. Ne4 d2 2. Nf6+ Kh8 3. Ne7 (if black did not have the pawn at this point, the game would be a draw because of stalemate) 3. ... d1=Q 4. Ng6# (see algebraic notation). If Black did not have the pawn move available, White could not force checkmate.