The author examines the scope and interrelation of two traditional notions concerning non-literary Latin: sermo uulgaris, or plebeius, and sermo familiaris, or cotidianus. While these are really disparate terms, the one designating a sociolect and the other a language register, the author maintains that the old confusion between Colloquial and Vulgar Latin is not merely due to flawed reasoning within an insufficient model of linguistic variation, but rather reflects a fundamental development that took place in the social history of Latin.