commonletter

Commonletter is a term used in text analysis and cryptography to denote the most frequently occurring letter in a given body of text. It is typically treated as case-insensitive and restricted to alphabetic characters, though some analyses may include accented letters as separate symbols or map them to base letters.

Computation of the commonletter involves counting occurrences of each letter and selecting the one with the highest count. Most implementations normalize to a single case, remove whitespace and punctuation, and may handle non-Latin alphabets differently depending on the language and purpose of the analysis.

The specific commonletter depends on language and genre. In English prose, the letter e is commonly the most frequent, often followed by t, a, o, i, and n, but results can vary with text type, size, and authorship. In multilingual corpora or specialized domains, other letters may dominate.

Applications of the commonletter include cryptanalysis, where knowledge of letter frequencies helps infer plaintext, as well as language identification, typographic research, and keyboard layout design. It also serves as a basic descriptor in text mining and data profiling to compare different corpora.

Limitations include sensitivity to sample size, genre, and language; short or atypical texts can yield unstable results. The commonletter is a descriptive metric that should be used with other frequency measures and contextual analysis to draw reliable conclusions.

See also: letter frequency, frequency analysis, cryptanalysis, text mining.