Giovanni Boccaccio (1313 – 1375) along with Dante (who went before him) and his contemporary Francis Petrarch (the source of the Clerk’s Tale of Griselde), is one of the three greatest and best known poets of the Italian fourteenth century. Chaucer certainly knew the writings of all three poets, and perhaps met both Petrarch and Boccaccio (more likely Petrarch, but potentially both).
Boccaccio’s Decameron, a long work compromising several shorter tales, is often thought to be the main stylistic influence on the Tales, though there is no evidence that Chaucer ever read it as an entire work. The Tales as a whole, however, do owe much to Boccaccio: the Knight’s Tale is based on Il Teseida and Chaucer’s “heigh style”, as Larry Benson has commented, “owes something to Boccaccio's attempt to emulate the classics in his own vernacular.” The Monk’s Tale draws on Boccaccio’s works in Latin, and the Clerk’s, Franklin’s, Merchant’s, Pardoner’s, Reeve’s and Shipman’s Tales all have analogues (or at the very least, stories with considerable similarities) in Boccaccio’s Decameron.
