Alpha-beta pruning
Alpha-beta pruning is a search technique that skips branches of the move tree that cannot change the minimax result, letting engines look much deeper using the same amount of computing time.
Also called: alpha beta search
A chess engine must examine a huge tree of possible moves. Alpha-beta pruning is a method that cuts away parts of that tree without changing the answer. While searching, the engine keeps two bounds: the best score the side to move can already guarantee, and the best score the opponent can already guarantee. If a line is shown to be worse than an option already available, the remaining moves in that line need not be examined, because a sensible opponent would never allow it.
The pruning gives exactly the same result as a full minimax search, only far faster. Its power depends on looking at promising moves first, so engines order moves carefully, trying captures and previous best moves before quiet ones. Good ordering lets the search go roughly twice as deep in the same time.
You do not need to run this by hand, but it is a useful picture of how good players think too. When you find that one reply refutes a candidate move, you stop analyzing that move and turn to another. That habit of discarding a line once it is refuted saves time and keeps your calculation focused.
A neighboring idea is Search depth. Search depth is how many moves ahead an engine has looked in a position, counted in half-moves called plies, and deeper searches generally give more reliable evaluations and moves.
A neighboring idea is Stockfish. Stockfish is a free, open-source chess engine developed by a worldwide community, widely regarded as one of the strongest engines available and used by many analysis websites and apps.
An engine can tell you what "Alpha-beta pruning" means for a position, but not which mistakes you keep making; a first lesson with George Li reviews your games and finds those.
Related terms
Minimax is a decision method in which one side tries to maximize the evaluation while the opponent tries to minimize it, forming the basis of how traditional chess engines choose moves.
Search depth is how many moves ahead an engine has looked in a position, counted in half-moves called plies, and deeper searches generally give more reliable evaluations and moves.
A chess engine is a computer program that searches positions and evaluates moves, usually playing and analyzing far stronger than any human, and it is used for analysis, preparation and play against a machine.
Stockfish is a free, open-source chess engine developed by a worldwide community, widely regarded as one of the strongest engines available and used by many analysis websites and apps.
An evaluation is a numerical judgment of who stands better in a position, usually given in pawns from White’s point of view, where positive favors White and negative favors Black.
The principal variation is the line of best play for both sides that an engine has found, shown as a sequence of moves and tied to the evaluation it reports.
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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A game review, a read on your level and a written study plan from George Li.
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A first lesson starts with your games: George reviews them and builds a study plan around what he finds.