My teacher today tried telling me Charlemagne signed his name with an "X" but I don't believe that. Evidence?

I know King John of England in 1215 signed the Magna Carta but since he was illeterate he used a giant X as his name. Charlemagne though spoke Latin, A germanic idiom, old french, and understood Greek. I know he read some books and tried writing in his old age but never got the hang of it exactly. She also said he had people read things to him because he couldn't read, and signed his name with an X, but I don't believe that at all. (I was doing a report on Charlemagne. That's how it came up - she's my french teacher, and taught history a long time ago, I asked a history teacher at my school, but they didn't know really, but said that they wouldn't think so. I also googled it, but I didn't get many results that were exactly easy... It doesn't look like he used an X though, but I want the opinion of someone who knows more about Charlemagne and King John.) Anyone know for sure with the source of a website that's legitimate and could tell me so I can know for sure?