obstacles in informtics...

[imlife]

In a book of informatics, I read "were we to have some prior information, according to which we had good reason to suppose the length to be close to nine inches (perhaps we had previously requested the wood to be chopped to that length), the probabilities of the lengths 8, 9, 10 inches would perhaps be 0.25 each, and the sum of all the others would be 0.25. The existence of this prior knowledge would somewhat reduce the quantity of information gained from the measurement, namely to 4/3 * log2(4) + 1/4 * log2(36) = 2.79 bits."
My question is: what doed the underlined 36 mean?
Would someone please give me detailed explaination? Thanks!