Michele Giannetti (Barga [Lucca] 1598–Florence 1666) was a Tuscan priest, a professor of theology at the University of Pisa, and an assessor of the Sant’Uffizio. In 1651 he published in Florence his Grammatica toscanalatina, a grammar of Latin written in the vernacular, today preserved only in three copies. As a preface to his Grammatica, Giannetti published an interesting and hitherto unknown text called Fabbrica del parlare e chiave delle lingue. This work proposes an innovative approach not only to the rules of Latin language, but also to the grammar of other languages, both dead and living. Giannetti outlines a new idea of grammar for both spoken and written languages, focusing on concepts such as the costruzione (syntax) and significazione (meaning) of sentences; while criticising previous works such as Kaspar Shoppe’s Mercurius bilinguis (1632) and quadrilinguis (1637), the text by Giannetti offers new perspectives on the learning of the vernacular in seventeenth-century Italy.