The numerical system we know from antiquity made its appearance for the first time in inscriptions of the 5th century B.C., but was obviously formed during the Archaic period. This system is based on a method of writing that was known in Assyria and Phoenicia, and is called acrophonic. That is, the number was written with the first letter of the word that indicated it: Π for pente, D for deka, H for hekaton, X for chilia, and M for myriads (tens of thousands). The units were shown with a vertical bar. Multiples of these numbers were indicated by repeating them and by inscribing the letter in question between the horns of the letter Π (when it was a question of a multiple of five): . Thus the number 1998 would have been written . This system was replaced in the first Imperial years with the alphabetic, the use of which continued into the Byzantine period.