Stylometry compares ways of writing: everyone has nearly unconscious habits (turns of phrase, recurring mistakes, punctuation, abbreviations). Paste an anonymous text on the left and a known text on the right: the tool counts each one's habits and measures how alike they are. The habit list is tuned for French texts.

In real life: forensic linguistics (the Unabomber case, the Tulle « poison pen » letters), the JGAAP and stylo (R) software. A single habit proves nothing; it is the build-up of rare, shared habits that points to an author, and the conclusion remains a presumption to be confirmed.