Move ordering

Move ordering changes the sequence in which a chess engine, a program that analyzes positions, examines legal choices. It does not remove a move merely because it looks weak beforehand. Instead, it tries to place first the candidates most likely to be strong, produce a quick refutation, or provide useful information.

Order matters because many searches use alpha-beta pruning, a method that stops examining a branch once that branch is proven unable to improve the current decision. If an excellent move appears early, it sets a demanding reference and many inferior replies can be rejected with little work. If the same move appears last, the engine may already have spent time on branches that later become irrelevant.

Common ordering signals

• The best line from an earlier search, especially during iterative deepening.
• A stored move found when the engine recognizes that it has already searched the same position.
• Captures that appear favorable, sometimes filtered with a fast estimate of the material exchange.
• Quiet moves, meaning moves without an immediate capture or check, that previously caused cutoffs in similar positions, according to internal records of earlier results.

In a complete fixed-depth search, a different order should lead to the same final result. What can change dramatically is the amount of work needed to reach it. Techniques such as principal variation search depend even more heavily on finding a good candidate early.