We consider a simple extension of the familiar Tower of Hanoi puzzle. There are three vertical pegs lined up in a row and n discs. The discs have holes in their centres so that they can be threaded on to the pegs. Initially, all n discs are placed on the left-hand peg in non-increasing order of radius to form a tower, as in Figure 1. The object of the game is to transfer the entire tower to the right-hand peg by moving discs from peg to peg, one at a time according to the rules:

• (1) only a disc at the top of a tower may be moved;

• (2) a disc must never be placed on top of a smaller disc.