It's a secret (9)

This is a Viginère square.  If you look closely you will see that it is simply a collection of all the Caesar shifts from 0 (or 26) - the "A" line, to 25 - the "Z" line.  How it became associated with Blaise de Viginère is a bit of a mystery, because he sure didn't invent it, but this was to be the basis of a new and for a long time, an unbreakable substitution cipher.  How - we'll see next time.