Editing Transposition table (section)

== Replacement strategies ==
A transposition table is a cache whose maximum size is limited by available system memory, and it may overflow at any time.  In fact, it is expected to overflow, and the number of positions cacheable at any time may be only a small fraction (even orders of magnitude smaller) than the number of nodes in the game tree.  The vast majority of nodes are not transposition nodes, i.e. positions that will recur, so effective replacement strategies that retain potential transposition nodes and replace other nodes can result in significantly reduced tree size.  Replacement is usually based on tree depth and aging: nodes higher in the tree (closer to the root) are favored, because the subtrees below them are larger and result in greater savings; and more recent nodes are favored because older nodes are no longer similar to the current position, so transpositions to them are less likely.

Other strategies are to retain nodes in the principal variation, nodes with larger subtrees regardless of depth in the tree, and nodes that caused cutoffs.