We all know about Arabic and Roman numerals. But what did the Greeks use? They liked their math and geometry, so they must have had some kind of number system.
Your question got me curious. Until a real answer comes along by somebody more knowledgeable than Wikipedia, check here and here.
Their letters also represented their numbers. There were four letters beyond what’s considered the greek alphabet as we know it today that encapsulated a couple extra numbers.
This site, while not perfect, gives a pretty good idea of how it works.
It should be noted that Greek astronomers often used the Babylonian numerical system instead (a sexigesimal, that is, base-60, positional number system), because calculations were much much easier, especially for large numbers. Ptolemy, an astronomer from around A.D. 150 introduced use of “omicron” for certain times when we use “zero.” This was not, however, “zero” in its formal, numerical sense, but more of a place-holder (such as the “0” in the number 230).
But Greeks generally used the Greek alphanumeric numerals to represent the sexagesimal digits; they didn’t take over the cuneiform numeral signs. So even the astronomers didn’t have a totally positional system.
I don’t believe that is true of the ones who worked in, say, Egypt. Indeed, Ptolemy couldn’t have used omicron for “nothing” if using the Greek alphabetic numbers because omicron already had a purpose. 