The Roman author and politician Marcus Tullius Cicero (106 B.C.–43 B.C) was an important role model for humanists. Cicero's opinions regarding the style and content of letters influenced many scholars. In fact, when, in the 1340s, Petrarch rediscovered a collection of Cicero's letters, he was inspired to start writing and collecting his own "familiar letters".
Cicero writing his letters, woodcut, 1547, unknown artist; source: Cicero, Marcus Tullius: Epistulae ad familiares, printed by Hieronymus Scotus (alias Girolamo Scoto), Venice 1547, Scan: Roman Eisele, http://commons.wikimedia.org/wiki/File:CiceroEpistulaeAdFamiliaresVenice1547page329Detail.jpg, public domain. ,