How undergarments (but probably also moms) improved medieval literacy

As more people migrated to urban centers in the 13th century, undergarment use went up. Thrown out pieces were an affordable substitute for parchment, and thus helped lower the cost of publishing. Comments here indicate that plenty of the surplus cloth arrived due to the Black Death.
However,

... even in countries where there were few or no towns and, therefore, schools, such as Iceland, the literacy rate continued to grow rapidly. “Many people learnt to read at home, usually from their mothers,” says Dr Mostert. “The role of female literacy in the home has been underestimated until quite recently.”