Frequencies of character pairs provide important clues for cryptanalysis. In practice you would try to obtain text samples from actual communications or, alternatively, use typical texts. This Demonstration visualizes frequencies of pairs of letters using words from the built-in dictionary in Mathematica. Tooltips give these frequencies.
A red disk indicates that a pair is more frequent than the reversed-order pair (i.e., a red disk at "ly" indicates this is more frequent than "yl"). Blue disks indicate the opposite, and purple disks indicate similar frequencies of both orderings for that pair. Reduce the displayed number of pairs by raising the threshold, which is the minimum number of words in which a given character pair has to be present in order to appear in the graphic.