 An early search paradigm called alpha–beta pruning, a system of defining upper and lower bounds on possible search results and searching until the bounds coincided, reduced the branching factor of the game tree logarithmically, but it still was not feasible for chess programs to exploit the exponential explosion of the tree.This led naturally to what is referred to as "selective search", using chess knowledge (heuristics) to select a few presumably good moves from each position to search, and prune away the others without searching.But chess is not a game given to topical inspection, and the goodness or badness of a move may not be determined for many moves into the game, so selective search often resulted in the best move or moves being pruned away.