Underpromotion
Underpromotion is promoting a pawn to a rook, bishop or knight instead of a queen, chosen because the lesser piece wins or draws more in the position at hand.
Most pawns that reach the last rank become queens, so choosing otherwise needs a concrete reason. The most frequent one is a knight. A knight moves differently from a queen and can give check or attack two pieces in a way a queen cannot from the same square. A second reason is to avoid stalemate, where promoting to a queen would leave the opponent with no legal move, though a rook or other piece keeps the game going.
A rook or bishop is occasionally chosen in endings where it keeps a draw or a win that a queen would spoil.
To spot an underpromotion, ask what each of the four pieces would attack on the promotion square, and whether any of them gives check. Compare that with the queen's effect. Do not forget that a pawn can promote on a capture, which changes the squares your new piece can reach.
Promoting on autopilot is how the chance slips by. Online players with automatic queening switched on, and over-the-board players short of time, sometimes take a queen out of habit and miss a stronger or even winning alternative.
Compare it with Royal fork. A royal fork is a fork that attacks the enemy king and queen at the same time, most often with a knight, so the king must move and the queen is lost.
Compare it with Stalemate trick. A stalemate trick is a resource for the side that is losing: it forces the stronger side into a position where the weaker side has no legal move and is not in check, so the game is drawn.
A neighboring idea is Knight fork. A knight fork is a move that places a knight on a square from which it attacks two or more enemy pieces at once, so the opponent cannot save them all.
Knowing "Underpromotion" helps most once you can spot it in a real game. A first lesson with George Li starts from your own games and finds the tactics you keep missing.
Related terms
A fork is a move that leaves one of your pieces threatening two enemy pieces together. Because the opponent gets only one turn to respond, something usually falls.
A knight fork is a move that places a knight on a square from which it attacks two or more enemy pieces at once, so the opponent cannot save them all.
A double attack is one move that creates two threats at once, whether they come from a single piece or from two, and whether they aim at pieces or at checkmate.
A stalemate trick is a resource for the side that is losing: it forces the stronger side into a position where the weaker side has no legal move and is not in check, so the game is drawn.
A combination is a forcing sequence of moves, usually built from sacrifices and checks, in which every move leaves the opponent few or no choices and the sequence ends in a concrete gain such as mate or material.
A royal fork is a fork that attacks the enemy king and queen at the same time, most often with a knight, so the king must move and the queen is lost.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
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