The problem for the Medieval Spanish scholars who were tasked with translating this material is that the letter sheen and the word shayun can’t be rendered into Spanish because Spanish doesn’t have that SH, that “sh” sound. So by convention, they created a rule in which they borrowed the CK sound, “ck” sound, from the classical Greek in the form of the letter Kai.
Later when this material was translated into a common European language, which is to say Latin, they simply replaced the Greek Kai with the Latin X. And once that happened, once this material was in Latin, it formed the basis for mathematics textbooks for almost 600 years.
But now we have the answer to our question. Why is it that X is the unknown? X is the unknown because you can’t say “sh” in Spanish. (Laughter) And I thought that was worth sharing.Terry Moore
Editor’s note, 2020. Another tribute to my now-defunct Tumblr account. This quote entry wouldn’t exist had NPR’s link rendered an image to go with the link itself. Having spent hours trying to do something today, I’ve now done something. Yay me.