The central thesis of this paper is that from the time of the Alexandrian conquests the Greek-speaking community was characterized by a diglossia in which the Attic dialect became the standard language, while a simplified version of this dialect served as the colloquial. On the basis of a comparison between the term ϰοινή in grammatical literature and the terms διάλεϰτος, συνήϑεια, and ἑλληνισμός, the following conclusions are drawn: 1) the grammarians used the expression ϰοινή διάλεϰτος for the Greek language in general; 2) this ϰοινή was for them their own language; 3) the ϰοινή contrasted with the dialects as regional varieties; and 4) the ϰοινή did not include the colloquial. From this it follows that the Classical language remained alive as the high variety of post-Classical diglossia, which was also used in a spoken form (the συνήϑεια of the grammarians). This means that the grammatical writings cannot be used as evidence of the development of the popular language in this period.