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The Best Move is a Search related issue.

The Best Move in the context of search algorithms like Alpha-Beta or Principal Variation Search, is found at the root or any other confirmed PV-node. It is that move which gained the maximum score, which is exact since it exceeds alpha but is still below beta and therefor not good enough to refute opponent's previous move, which may become his best move and part of the principal variation as well.

Best so far

If an alpha increase occurs, the recently searched move is the best so far and needs to be stored on an explicit or implicit stack, which also might be the triangular PV-Table if available.

TT-Entry

After all remaining moves were searched at a PV-node, the best move found is stored inside the transposition table together with its associated values and flags.

PV-Table

If using a triangular PV-table to collect the principal variation, one has to store the best move (so far) anyway - immediately after an alpha increase occurs, to append the PV of ply + 1, which would otherwise void while searching further.

Cut-Node

At Cut-nodes, if a move fails high, it is not necessarily the best move, but a refutation move, good enough to refute opponents previous move. Since it is the best move found so far, in the context of transposition table, it is often flippant mentioned as best move as well.

All-Node

At All-nodes there is no best move found at all, despite all childs were considered if not pruned. While there is a maximum score in the arithmetical sense, all scores below or equal to alpha are upper bounds and not comparable to determine a best move.

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