3. Letters showing a wide variety in their contact-letters are usually vowels.
4. In repeated digrams, one letter is usually a vowel.
5. In reversed digrams, one letter is usually a vowel.
6. Doubled consonants are usually flanked by vowels, and vice versa.
7. It is unusual to find more than five consonants in succession.
8. Vowels do not often contact one another. If the letter of highest frequency can be assumed as E, any other high-frequency letter which never touches E at all is practically sure to be another vowel, and one which contacts it very often cannot be a vowel. (This will apply equally to other vowels, wrongly assumed as E.)
With a text of reasonable length, say 150 letters, it is sometimes possible to determine with certainty just which of the cryptogram-letters represent the six vowels; with shorter cryptograms, we can usually find four; sometimes only three. But once the separation has been made, individual vowels can usually be established as follows:
The most frequent one is ordinarily E. The one which never touches it is most likely to be O. Both of these are very freely doubled, and for that reason are often confused with each other, but seldom with any other vowel. They rarely touch each other.
The vowel which follows E and almost never precedes it, is A.
The vowel which reverses with it is I.