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?

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    I edited your question to something a little better suited for the site and voted to reopen it. If it differs too much from what you meant, feel free to edit further. – Kareen Jun 8 '14 at 16:38
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    @Kareen The use of Roman numerals is general knowledge and have nothing particular to the French language. They are used for dates and yeafrs in a lot of European countries. English uses them a lot as well, see relevant article in Wikipedia for instance. – Laure SO - Écoute-nous Jun 9 '14 at 6:20

In French, centuries are usually indicated with roman numerals. In this case, XX is 20. “XXe siècle” means “20th century”.

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