I came about the following:
Au XXe siècle
Is XX representing some unknown value for the century? How do you know what it is? Does it mean something else entirely?
closed as off-topic by Laure, Alexis Pigeon, JE SUIS CHARLIE, Ri49, Zistoloen Jun 10 '14 at 10:59
This question appears to be off-topic. The users who voted to close gave this specific reason:
In French, centuries are usually indicated with roman numerals. In this case, XX is 20. “XXe siècle” means “20th century”.