I've been reading Giovanni Boccaccio's Decameron off and on lately, and one of the things I love about my copy (it's the Penguin Classics edition, translated by GH McWilliam) is the constant footnotes. They tell me everything from "yeah, that character was a real person, even though nothing like this actually happened to them" to "this entire story is a folktale of Indian origin." From a folkloristic perspective, that last kind of comment is always fascinating, and makes me want to know more.
The Decameron is part of a rich tradition of collections of stories on various themes, which often borrow from one another and consequently show up in other forms in other places. It turns out a surprising number of the stories originate in Eastern sources. Notable among these is the Panchatantra, a collection of animal fables and other tales written in Sanskrit in the 3rd century BCE; it's sometimes surprising how little the plots changed in the millennium and a half between this origin and the Decameron. Stories from the Panchatantra also appear in other places, notably including the Spanish El Conde Lucanor, the fables of Aesop, and the Thousand and One Nights.
Boccaccio, obviously, did not have the Panchatantra available to him as a direct source. Rather, the stories must have either passed west into Europe via the Silk Road, or found their way into Italy from their trade with the Middle East during the late medieval period. In either case, they were passed from author to author by a mix of oral and literary transmission. Boccaccio does a masterful job of adapting them to his audience, as is seen on those rare occasions where it is abundantly clear what textual source still known to us he was working from. In other instances, no immediate precursor is known, and it is unclear what features are of the compiler's own devising and what innovations arose in the oral tradition as the story made its way to his pen.
Stories in Boccaccio found in the Panchatantra include II.2, III.2, IV.2, and VII.8. However, Sanskrit-language antecedents are also known for III.5, III.9 (which is also the basis of Shakespeare's All's Well That Ends Well), IV.10, VI.10, VII.4, VII.6, and X.5; Persian origins are known for V.9 and VII.9, and the Thousand and One Nights contains versions of I.5, II.7, IV.2, and VII.9. In all, sixteen of the hundred stories in the Decameron are known to be derived from Oriental sources.
It's also worth noting that Chaucer's Canterbury Tales contains versions of several stories also found in the Decameron, among them VII.9 (known from Persian sources as well as the Thousand and One Nights); evidently the popularity of this story, in which an adulterous wife and her servant convince her husband that his eyes deceive him as a result of a magical pear tree, transcends cultural boundaries. (Heather Dale recorded an excellent song based on Boccaccio's version of the story for her album The Road to Santiago, proving that it has not lost its capacity to amuse modern audiences.) Chaucer also uses versions of VIII.1 and IX.6.
One of the most interesting examples of a Panchatantra story updated for the Decameron is that of King Agilulf (III.2), which may also have an antecedent in Herodotus' Histories. (The historical Agilulf, King of Lombardy, lived in the 6th and 7th centuries, long after the earliest versions of this story were recorded.) One of the grooms at Agilulf's stables makes love to the queen in the guise of the king; when the real king arrives afterward, she comments that it is not usual for him to come to her twice in one night, and so he realizes what has happened. After the two of them have sex, Agilulf goes to the stables and locates the one servant whose heart is beating rapidly from the exertion and cuts that man's hair. Once the king has left, however, the servant in question gets up and cuts the hair of all the others, so that when they are all called before the king in the morning he is unable to tell which one he had marked as the guilty party. Agilulf realizes that he cannot explain what has happened without bringing dishonor to both himself and his wife, so he dismisses them and keeps the secret for the rest of his days. The ending, in which the king is praised for his discretion in protecting the honor of the queen, is distinctly a product of the nascent Italian renaissance; maintaining honorable appearances even at the expense of bringing wrongdoers to justice fits with the preferences of Boccaccio's audiences, and is not emphasized in earlier versions of the story.
Even outside the stories derived from oriental sources, the Decameron is rife with stories adapted from other sources, both oral and literary. Many are taken from Latin texts which Boccaccio (a well-educated and well-read man) was familiar with, while others (such as II.2, a popular 14th-century joke) derive from the oral tradition in Italy at that time. But in all cases, as in the example of the story of King Agilulf, the talented storyteller knows his audience's preferences, and so Boccaccio makes a point of adapting his sources to fit the context of his work; the only passage blindly plagiarized is in fact in his introduction, when he purports to give a firsthand account of Florence during the summer of the Black Death which he has in fact lifted from an eighth-century chronicle. Although Boccaccio was formally educated and the presentation of the stories in the Decameron is self-consciously literary, such adaptations are part of the expertise of a talented folk storyteller, and between the transmission of many of the stories to Boccaccio and the confidence with which he presents his own versions, he is a delightful source for well-told versions of the folktales of the early Italian renaissance.