Second Troitsky line

The second Troitsky line belongs to the rare two knights versus pawn ending. Two knights cannot force mate against a lone king, but the defender's pawn can supply a legal move and remove a stalemate defense. Stalemate is an immediate draw when the player to move has no legal move and the king is not in check.

The basic technique is to blockade the pawn with one knight. While that knight holds the pawn, the king and the other knight drive the opposing king toward a mating area. Only near the end does the blockading knight leave its post and join the attack. Capturing the pawn too early usually destroys the forced mate.

The original Troitsky line identifies blockade squares from which a broad theoretical win exists, but some winning procedures take more than fifty moves. The second line is a stricter boundary designed to account for the practical rule: a player may claim a draw after fifty moves by each side without a pawn move or capture. A pawn move or capture restarts that count.

It is not a straight geometric line or a guarantee detached from the rest of the board. It is a map of blockade squares that must be read together with both kings, the second knight, the side to move, and the current move count. Zugzwang is also central because the winning method relies on leaving the defender a pawn move at the right moment.