The figures 1, 2, 3, 4, etc. So called because they were introduced into Europe (Spain) by the Moors or Arabs, who learnt them from the Hindus. Far more important than the characters, is the decimalism
of these figures: 1 figure = units, 2 figures = tens, 3 figures = hundreds, and so on ad infinitum.
The figures i, ii, iii, iv, v, vi, vii, viii, ix, x, etc., are called Roman figures. The Greeks arranged their figures under three columns of nine figures, units, tens, and hundreds, and employed the letters of the alphabet. As there are but twenty-four letters, a sansculotte letter had to be introduced into each column. In the units column it represented 6, and was called episemon. In the tens column it represented 90, and was called koppos. And: in the third column it represented 900, and was called sanpi. Thousands were represented by a dash under some letter of the first three columns.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894