Although there are 13 circled and 12 uncircled nonterminal nodes in the tree, White has 16,384 pure strategies, while Black has only 81. A path between nodes a and a in a tree is simply a sequence of contiguous edges joining a and a . A rooted tree has a natural partial ordering of nodes, known as beyondness. The subgame corresponding to a node a consists of a itself, together with all the nodes and edges lying beyond a . The depth of a node is the number of moves required to go from that node to the most distant of the terminal nodes that lie beyond it.
Terminal nodes are subgames of depth zero.
Every node of a game tree represents a subgame of the original, and every move made by either player can be regarded as the opening move of some subgame. The effect is always to transfer play to a strictly shallower subgame. Subgames of depth 0 are just outcomes, and the rules of the game automatically classify them as W, B, or U. A subgame S of depth d merits the same classification as the most favorable, to the player making the opening move in S, of the shallower subgames to which he or she can directly transfer play. The given classification of subgames of depth 0 can thus be extended, first to subgames of depth 1, then to subgames of depth 2, and so on. In principle, even chess can be solved in this manner.