Boccaccio borrowed the plots of almost all his stories (just as later writers borrowed from him). Although he consulted only French, Italian and Latin sources, some of the tales have their origin in such far-off lands as India, Persia, Spain, and other places. Some were already centuries old. For example, part of the tale of Andreuccio of Perugia (II, 5) originated in 2nd century Ephesus (in the Ephesian Tale). The frame narrative structure (though not the characters or plot) originates from the Panchatantra, which was written in Sanskrit before AD 500 and came to Boccaccio through a chain of translations that includes Old Persian, Arabic, Hebrew, and Latin. Even the description of the central current event of the narrative, the Black Plague (which Boccaccio surely witnessed), is not original, but based on the Historia gentis Langobardorum of Paul the Deacon, who lived in the 8th century.
Some scholars have suggested that some of the tales for which there is no prior source may still not have been invented by Boccaccio, but may have been circulating in the local oral tradition, with Boccaccio simply the first person known to have recorded them. Boccaccio himself says that he heard some of the tales orally. In VII, 1, for example, he claims to have heard the tale from an old woman who heard it as a child.
The fact that Boccaccio borrowed the storylines that make up most of the Decameron does not mean he mechanically reproduced them. Most of the stories take place in the 14th century and have been sufficiently updated to the author's time that a reader may not know that they had been written centuries earlier or in a foreign culture. Also, Boccaccio often combined two or more unrelated tales into one (such as in II, 2 and VII, 7).
Moreover, many of the characters actually existed, such as Giotto di Bondone, Guido Cavalcanti, Saladin and King William II of Sicily. Scholars have even been able to verify the existence of less famous characters, such as the tricksters Bruno and Buffalmacco and their victim Calandrino. Still other fictional characters are based on real people, such as the Madonna Fiordaliso from tale II, 5, who is derived from a Madonna Flora who lived in the red light district of Naples. Boccaccio often intentionally muddled historical (II, 3) and geographical (V, 2) facts for his narrative purposes. Within the tales of the Decameron, the principal characters are usually developed through their dialogue and actions, so that by the end of the story they seem real and their actions logical given their context.
Another of Boccaccio's frequent techniques was to make already existing tales more complex. A clear example of this is in tale IX, 6, which was also used by Chaucer in his "The Reeve's Tale", which more closely follows the original French source than does Boccaccio's version. In the Italian version, the host's wife (in addition to the two young male visitors) occupy all three beds and she also creates an explanation of the happenings of the evening. Both elements are Boccaccio's invention and make for a more complex version than either Chaucer's version or the French source (a fabliau by Jean de Boves).