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The frequency of letters in text messages has often been studied for use in cryptography, and frequency analysis in particular. An exact analysis of this is not possible, as each person writes slightly differently, and the amount of writing in a language is too vast to allow anything more than a statistical sample of a language to be analysed. Linotype machines used the ordering etaoin shrdlu cmfwyp vbgkqj xz based on what was believed the order of frequency of use was in the English language at the time the machine was invented. Likewise, Modern International Morse code was designed by arranging the letters of the alphabet in a binary tree in order of their perceived frequencies, et ianm surwdkgo hvf*l*pjbxcyzq**, such that the shortest codes were assigned to the most frequently used letters.

More recent analyses show that letter frequencies, like word frequencies, tend to vary, both by writer and by subject. One cannot talk about x-rays without using frequent Xs, and different authors have habits which can be reflected in their use of letters. American English has more 'z's than British English, because American English uses the suffix "-ize" while British English also uses "-ise".

Everyone writes differently – for example, Hemingway's writing style is quite visibly different from Faulkner's. Letter, bigram, trigraph, word frequencies, average word length and average sentence length can be calculated for specific authors, and used to prove or disprove authorship of texts, even for authors whose styles aren't so divergent.

A precise average usage can only be gleaned by analyzing usage in a large mass of representative text; since the availability of modern computing and collections of large text corpora, such calculations are nearly trivial.

Relative frequencies of letters in text.

Relative frequencies ordered by frequency.

(source)

• Corpus linguistics
• ETAOIN SHRDLU
• Frequency analysis (cryptanalysis)
• Linotype machine
• Most common words in English
• Scrabble

• A site with content of Cryptographical Mathematics by Robert Edward Lewand
• Some examples of letter frequency rankings in some common languages