A to Z Book of Me Challenge X ( Marks the spot )

X marks the spot...

Did any of our ancestors not know how to write, and therefore instead of
their name, they would put an x there instead.?
Or...
due to an injury or maybe an illness they were unable to write their name,
so put an x

So far with the documents that I have , my ancestors have actually
written their name... but I don't really have anything that goes really, really far back.

I think that before the early 20th century, it would be quite common for most
people to not know how to write.
If we do come across that with our own ancestors, to know if they could write
at all, we could search other documents, eg: wills, birth certificate etc and see
if they wrote their name on that, or just put an x instead.