Like Roman numerals, their system borrowed letters; like the Arabic numerals we still use, it only needed one symbol for each decimal place. In the 6th century BCE, the Greek alphabet used 24 letters.

A math historian explores how "x" came to stand in for an unknown quantity. ... He called the unknown “arithmos” and used an archaic Greek letter similar to “s” for the unknown.

You might be most familiar with x from math class. Many algebra problems use x as a variable, ... They represented this sound by the Greek letter χ, which later became the Latin x.

An M with a small letter or set of letters above it stood for tens of thousands and higher. (Confusingly, in this instance the M represented the Greek ancestor of “myriad,” not the number 40.) ...