Everyone thinks the printing press led to increased literacy among the average man in the middle ages, but that just might not be the case. Dr Marco Mostert a historian from Utrecht University is instead suggesting that the availability of cheap paper was the main reason more reading material became available. While this isn’t surprising the source of the new cheap paper is. It seems that, according to Dr. Mostert,
“These rags came from discarded clothes, which cost much less than the very expensive parchment which was previously used for books. In the 13th century, so it is thought, as more people moved into urban centres, the use of underwear increased – which caused an increase in the number of rags available for paper-making.”
For more random medieval literacy facts (none as exiting as this though) check out the EurekAlerts press release.
In case you’re wondering what medieval underwear looks like here’s an example.
This is a reproduction created by copying paintings by Antonello Da Messina and Piero della Francesca

-update-
According to HR Downs:
True enough that an overabundance of clothing triggered the massive surge in printed materials during the Middle Ages. However, the surplus was not limited to underwear, it was all manner of clothing. The Black Death killed so many people that enormous piles of clothing accumulated practically everywhere in Europe. Suddenly the raw material for rag paper became available to everyone left. This phenomenon is documented in Barbara Tuchman’s masterful history of the Black Death “A Distant Mirror: The Calamitous 14th Century.