The partial orders and preorders that are being discussed are not the order that you write the elements inside braces. Writing a set as a list of elements inside braces is not intended to imply an order. But the order you are talking about (letters ordered alphabetically and numbers ordered in the normal way, and no comparison between the two types) is a valid partial order, and it has incomparable elements. In particular, \\(b\\) and \\(3\\) are incomparable.