In cards, why are princes called Jacks?

Today is the first day of school in our district. Our children unschool - meaning they homeschool by exploring their interests vs. following a set curriculum - but this is the first question that sparked our formal school year for 2009-2010:

Nate asked, "In cards, why are princes called Jacks?"

The answer turned out to be pretty surprising. According to the International Playing Card Society, the Jack is not a prince at all, but slang for "male servant" or "commoner". Originally the Jack was called the Knave. To learn more about the changing face of playing cards (or anything related to cards, really) check out their website for more information:

International Playing Card Society