in Greek numerals, is a sign standing for a numeral. Thus, episemon bau generally called Fau, Episemon, stands for 6, and iota-episemon for 16. There are two other symbols— viz. koppa for 90, and sampi [san-pi] for 900. The reason is this: The Greek letters were used for numerals, and were ranged in three columns of nine figures each; but 24 letters will not divide by 9, so the 3 symbols, episemon, koppa, and sampi were added to make up 3 × 9. Col. 1, from 1 to 20; col. 2, from 20 to 100; col. 3, from 100 to 1,000.
Bau and Fau are identical, the B or F being the dijamma. Thus oinos (wine) was pronounced Foinos, called in Latin Vinum, and öon (an egg) was pronounced Ofon, in Latin Ovum.
A dash under a letter multiplied it a hundredfold. Thus, = 1, but =000. For intermediate figures between full tens a mark was made above the unit. Thus (iota = 10; but = 10 + 1 = 11, = 10 + 2 = 12; = 10 + 3 = 13, and so on.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894