The **Shannon number**, named after **Claude Shannon**, is a conservative lower bound (not an estimate) of the game-tree complexity of chess of **10 ^{120}**, based on an average of about

**1000 possibilities**for a

**pair of moves**consisting of a move for White followed by one for Black (

**two plies**), and a typical game lasting about

**40 such pairs of moves**.

The Shannon number refers tothe number of possible chess games ( 10)^{3}^{40} = 10^{120} |

If you think that this is not impressive enough, please keep it in mind that the **total number of atoms** in the **observable Universe** as we know it, is estimated to be ca. **10 ^{80}**. WOW!!!

Kind of more recently, **Victor Allis** , a Dutch computer scientist working in the artificial intelligence field, estimated the **game-tree complexity** of chess to be **at least 10 ^{123}**, based on an average branching factor of

**35**and an average game length of

**80 plies**.