"X Marks the Spot"

I've long heard about people marking X's for signatures when they couldn't read or write, and while I supposed it was true, I thought it was a bit of a cliché.

But I've just run across it in a 1753 Drouin marriage record for my 1st cousin 8x removed, Scholastique Lemay (b. 1731) to Joseph Bourgoin:

Add 29 Oct 1753 marriage record for Scholastique Lemay and Joseph Bourgoin.

For both of their signatures there's an X with a notation "marque de...".   I've looked at several hundred of these records (probably a few thousand by now), and that's the first time I've encountered this.

What's also strange is that while MOST of the marriage records have SOMEONE signing as a witness, it's not always the husband and wife (which I suppose in part would be if they were illiterate), and sometimes there are no signatures at all, though typically there's at least one by the priest.

Sometimes the space for the signatures take up most of the page.