Character Frequency Counter

In short A character frequency counter shows how often each letter, digit and symbol appears, with percentages, alongside the standard English baseline: E at 12.7%, T at 9.1%, A at 8.2%, O at 7.5%, I at 7.0% and N at 6.7%. Comparing your text against those numbers is the point. Ordinary English prose of any length converges on that distribution quickly, so a text that does not is telling you something — it may be in another language, it may be code or data rather than prose, or it may be a substitution cipher, which is exactly how frequency analysis breaks one. The technique is nearly a thousand years old and still the first thing anyone tries on an unknown cipher, because a simple substitution changes which symbol carries 12.7% without changing the fact that one of them does. For ordinary writing the use is narrower but real: checking which characters a font needs to cover, finding stray control characters in imported data, or confirming that a supposedly ASCII file really is one.