Plain text messages often have properties that aid cryptanalysis. For the purpose of explanation, consider messages written in English and encoded in ASCII text files (other file types have other regularities). A few concepts are general and important in understanding plain text regularities. These concepts are entropy, rate-of-language, and unicity distance. The significance of these concepts is that statistical regularities in plain texts are nearly as helpful for cryptanalysts as would be actually knowing the exact messages in question.