The following is taken from H.R. Smyth, Greek Grammar (1980), pp. 103-104, and M.N. Tod, "The Alphabetic Numeral System in Attica,", Annual of the British School at Athens 45 (1950) 126-139, and A.G. Woodhead, The Study of Greek Inscriptions, Oxford, 1981 (2nd ed.), 111-112.
Numerals of two types were used on Greek inscriptions .
1. Acrophonic numerals, which were never used as ordinals, were used
to denote cardinal numbers and units of value, weight and measure. (see
the page Acrophonic Numerals)
2. The alphabetic system (sometimes called "Milesian" numerals).
The alphabetic system was used for ordinal as well as cardinal numerals, for dates (day of month; length of time), for money, for distances, and as numeral adjectives (first, second, etc.). To mark off the numbers 1-999, modern print uses a stroke above and to the right of the letters, for 1000 and higher a stroke below and to the left. (These conventions were acquired from Byzantine inscriptions.)
Nine letters were needed for the units, nine for the tens, and nine for the hundreds. This requires 27 letters. Since the Greek alphabet has only 24 letters, three disused letters were pressed into service, one in each group. 6 is , (digamma or stigma), 90 is (koppa), 900 is (sampi).
A quasi decimal system is used. For example, eleven (11) is ten followed by one, . The numerals are arranged in descending order of value. This rule is observed everywhere in numbers exceeding 1000, though in some parts of the Greek world examples of a reversed or mixed order in the representation of numbers below 1000 are very common. For ten thousand and above, the acrophonic M for was used. To distinguish it from the M which represents 40, and to show what multiple of a thousand it represented, a smaller letter was placed above it. Thus =10,000; =20,000.