Nollasiirto

Nollasiirto, Finnish for “null move,” is a term used in computer chess to describe a search optimization technique that speeds up alpha-beta pruning. It is applied during game-tree search by evaluating the effect of a hypothetical move in which the side to move passes its turn to the opponent, effectively making a null move. The engine then searches from that position with a reduced search depth. If the null-move evaluation exceeds the current beta bound, a cutoff is performed at the original node, allowing the engine to prune the remaining moves. The goal is to quickly detect when the opponent has a clearly strong response and to reduce the number of branches that must be explored.

In practice, nollasiirto is used with a depth-reduction parameter, often lowering the search depth by one ply after a null move. It is a heuristic and is not guaranteed to be sound in all positions. Special care is taken in positions where the player is in check, where zugzwang can occur, or in endgames with tactical resources, to avoid incorrect pruning. Variants exist that adjust the reduction based on move ordering, history, or the presence of checks.

Nollasiirto has become a standard feature in modern chess engines since the 1990s, complementing other optimizations such as iterative deepening and transposition tables. It is discussed in the broader literature as null-move pruning. See also null-move pruning.