Minimal Window

Once the first move from a position in the tree has been searched, the program applies a minimal window search to the positions resulting from the remaining moves. Rather than search the new position with a window of (-beta, -alpha), a window of (-alpha-1, -alpha) is used. This way it can be quickly shown if a move leads to an inferior variation. If the value returned by the minimal window search falls within the alpha-beta window then the sub-tree must be searched with the window (-beta, -alpha) to determine the correct value.