When dealing with single codewords, we can find the average length of a codeword.
However, if we have knowledge that we are dealing with groups of codewords, this average length might be less.

Given code symbols A, B…

First/no extension - S^1 -> A, B Second extension - S^2 -> AA, AB, BB, BA Third extension - S^3 -> AAA, AAB, ABA, ABB, BAA, BAB, BBA, BBB

The n‘th extension will have a^n codes, where a is the number of single symbols.