Peggy? Jack? Polly?

Here's an interesting article that was linked in the comments at the Baby Name Wizard blog.

http://www.omaha.com/index.php?u_page=1219&u_sid=10304129

The author is generally quite knowledgeable about names and naming conventions. I've heard plenty of people express surprise at how Jack and John, for example, or Peggy and Margaret, are related. I honestly thought that was something everyone knew. Medieval nicknames! This article spells it all out.