Late Move Reductions

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Late Move Reduction (LMR) is a non-game specific search method which attempts to examine a game search tree more efficiently. It uses the assumption that good game-specific move ordering causes a program to search the most likely moves early. If a cut-off is going to happen in a search, the first few moves are the ones most likely to cause them. In games like chess, most programs search winning captures and "killers" first. LMR will reduce the search depth for moves searched later at a given node. This allows the program to search deeper along the critical lines, and play better.

Most chess programs will search the first several moves at a node to full depth. Often, they do not reduce moves considered to be very tactical, such as captures or promotions.

For more information see An Introduction to Late Move Reductions