The “próstaja mova” is one of the written languages used by both Ukrainians and Belorussians during the 16th and 17th centuries. In this article it is argued that its name is based on a calque of German Gemeinsprache, die gemeine Sprache, a term from the Reformation age. The „prostaja mova” was based on the Ruthenian (Ukrainian and Belorussian) chancery language and developed into a literary language because of its growing polyfunctionality, its increasingly superregional character, and its stylistic variability. The norms of the “prostaja mova” were based on its common usage, not on codification. We discuss the role of Church Slavonic and Polish elements on the different levels of this language and try to show that a “prototypical” text written in the “prostaja mova” was a translation from a real or only virtual Polish text, consisting in the “Ruthenization” of its phonology and morphology and, if it was a written text, in a change of the alphabets - the lexicon and the syntax, instead, remained mainly on a Polish basis. Until the 18th century the Polish language itself had gained so much importance among the Ruthenian gentry that the “prostaja mova” had lost its main addressee and was restricted only to some homiletic and cathechetic works for the common people of the Greek-Catholic Church.