The Shannon number is the estimate 
for the number of possible move sequences in chess
associated with Shannon's (1950) discussion of exhaustive computer play. Shannon
estimated roughly 30 legal choices at a typical position, or about 
choices for a pair of moves consisting of one move by White
and one by Black. Taking a typical game to last 40 such pairs gives
 |
This is an order of magnitude estimate based on typical play, rather than an exact enumeration of all legal chess games or a proved lower bound for their number. It also counts move sequences rather than distinct board positions.
In graph theory, the name Shannon number is also used for the Shannon capacity of a graph (Alipour et al. 2024). This is a different quantity from the chess estimate.
