The acrophonic system was replaced by a new alphabetic system, sometimes called the Ionic numeral system, from the 4th century BC. 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: fau ϝ, (also used are [Sigma (letter)|sigma] ϛ or, in modern Greek, ΣΤ) for 6, koppa ϟ for 90, and sampi ϡ for 900. To distinguish numerals from letters they are followed by the "keraia" (Greek "—insect antenna"), a symbol similar to an acute sign (Unicode U+0374).