The earliest system of numerals in Greek was acrophonic, operating much like Roman numerals with the following scheme: Ι = 1, Π = 5, Δ = 10, Η = 100, Χ = 1000, and Μ = 10000.
Starting in the 4th century BC, the acrophonic system was replaced with a quasi-decimal alphabetic system, sometimes called the Ionic numeral system. Each unit (1, 2, ..., 9) was assigned a separate letter, each tens (10, 20, ..., 90) a separate letter, and each hundreds (100, 200, ..., 900) a separate letter. This requires 27 letters, so the 24-letter Greek alphabet was extended by using three obsolete letters: digamma (ϝ) for 6, qoppa (ϟ) for 90, and sampi (ϡ) for 900. An acute sign (´) is used to distinguish numerals from letters.
The alphabetic system operates on the additive principle in which the numeric values of the letters are added together to form the total. For example, 241 is represented as σ´μ´α´ (200 + 40 + 1).