Historically, there were two systems for writing numbers in Greek. The acrophonic system used the first letter of the numeral name to represent some numbers. So the number 10, deka, was written using the symbol ∆, which represented the sound [d]. Similarly, π represented both the sound [p] and the number 5, called pente in Greek. Obviously every number could not be represented in this way; the names of multiple numbers began with each letter. To solve this problem, some numbers from 1-9 were not written with an alphabetic letter but by adding together the symbols for 1 (Ι) and 5 in appropriate combinations so that πΙΙ represented the number 7. Larger numbers used composite symbols, so the number 50 combined the π (5) symbol and the ∆ (10) symbol. Acrophonic numbers were mainly used in Athens and are now obsolete, although they provided the parent system on which Roman numerals, which are still used, were based.
The alphabetic system was more widely used, and provided the basis for Cyrillic and Hebrew representation of numbers, among others. This system assigned a numerical value of units, tens or hundreds to each letter of the alphabet. This required twenty-seven letters (1-9, 10-100 in tens, and 100-900 in hundreds): the twenty-four letters currently in use, plus three more which have since become obsolete. A superscript mark called keraia was written to the right of the symbol to indicate that a numerical, rather than alphabetic, value was intended. Larger numbers from 1,000 to 999,999 were written using the same 27 symbols, but with a left keraia mark in front to indicate that the symbol was being used to represent multiples of 1,000. So θʹ represented 9, and ͵θ represented 9,000. To combine units, for example to write the number 9,370, the symbols for 9,000 (͵θ), 300 (τʹ), and 70 (οʹ) were combined as ͵θτοʹ. The alphabetic system is still in occasional use in Greek texts, especially in academic papers for page numbers, plate / illustration numbers etc.
Latin numbers are used almost universally in Greek writing today.