They are often used in natural language processing for performing statistical analysis of texts and in cryptography for control and use of ciphers and codes.
See results of analysis of "Letter Frequencies in the English Language".
Context is very important, varying analysis rankings and percentages are easily derived by drawing from different sample sizes, different authors; or different document types: poetry, science-fiction, technology documentation; and writing levels: stories for children versus adults, military orders, and recipes.
Typical cryptanalytic frequency analysis finds that the 16 most common character-level trigrams in English are:[1][2] Because encrypted messages sent by telegraph often omit punctuation and spaces, cryptographic frequency analysis of such messages includes trigrams that straddle word boundaries.
[4] The sentence "the quick red fox jumps over the lazy brown dog" has the following word-level trigrams: And the word-level trigram "the quick red" has the following character-level trigrams (where an underscore "_" marks a space):