The traditional rule for quantity symbols in math is to use first letters of Latin alphabet for constants, and last letters for variables.
I just wonder why the two most used variable symbols, $x$ and $y$, are not $y$ and $z$. For example, why don't we see more often: $y=f(z)$ or $z=f(y)$. It seems Greeks had also zeta in their alphabet, which was borrowed by Romans.
I searched in A History of Mathematical Notations, a book referenced in another question, but I wasn't able to locate relevant explanations due to the mass of information it contains. However in history of Z letter, I saw Z was removed for some centuries, perhaps upon request of Roman censor Appius Claudius. Could that be the reason for not using $z$?