The same two observations will apply to the vowel O; but a distinction occurs when the vowel U can be found; this vowel precedes E and follows O.
The only vowel-vowel digrams of any real frequency are OU, EA, IO.
Three vowels found in succession may represent IOU, EOU, UOU, EAU.
As to identification of the consonants: Those letters still remaining in the high-frequency section of the frequency count will usually include T N R S H. Of these, the most easily identified is H, which precedes all vowels and seldom follows one; it may be identified often as part of repeated sequences TH, HE, HA.
Next to H, the most recognizable of the consonants, aside from frequency, is probably R, which reverses freely and indiscriminately with all vowels, and has a strong affinity for other high-frequency letters.
The consonant T can usually be identified by its frequency, by its tendency to precede vowels rather than follow them, and by its almost inevitable combination with H on more than one occasion. It is also notably difficult to distinguish from the vowels.
The letter N has characteristics which are to some extent the opposite of those mentioned for H; it prefers to follow vowels and precede consonants, and, to a lesser extent, the same is true of S, according to some charts. However, N, S, and T are all readily reversible with vowels, and are sometimes hard to tell apart.
The only frequent reversals of two consonants are ST-TS and RT-TR.
The doubles TT and SS are among the most frequent in the language.
Having this information, together with what we know of frequent digrams and frequent trigrams and very common short words, we are well armed against the longer cryptograms. Those which are shorter will give more trouble; but it takes a very short cryptogram indeed to be really resistant.