Historical origin of the letter "x" in French spelling

[Adam Kim]

I noticed the letter "x" did not exist in the general Classical Latin or Vulgar Latin orthographies maybe except for Greek loanwords.

 

How come French has the letter "x" as in "aux" or "Aix". It seems there is a vague pattern.

[Jim A.]

X was a shortcut for US, in old transcriptions of Latin, where that combination of letters was very common. Scribes later used it in old French for the same reason, in words ending in US—

Chevaus "horses" —> chevax
Aus "to the (m. pl.) —> ax

Over time, the U was reintroduced into the spelling and the X kept as the plural marker.

Other than in those cases, the X was taken directly from the Greek or Latin words. (Latin had in fact adopted X, Y, and possibly Z by the time Old French came about)