Abjadi order

The Abjad numerals are a decimal numeral system in which the 28 letters of the Arabic alphabet are assigned numerical values. They have been used in the Arabic-speaking world since before the eighth century when Arabic numerals were adopted. In modern Arabic, the word abjadīyah means 'alphabet' in general.

In the Abjad system, the first letter of the Arabic alphabet, alif, is used to represent 1; the second letter, bāʾ, is used to represent 2, etc. Individual letters also represent 10s and 100s: yāʾ for 10, kāf for 20, qāf for 100, etc.

The word abjad (أبجد) itself derives from the first four letters (A-B-G-D) in the Phoenician alphabet, Aramaic alphabet, Hebrew alphabet and other scripts for Semitic languages. These older alphabets contained only 22 letters, stopping at taw, numerically equivalent to 400. The Arabic Abjad system continues at this point with letters not found in other alphabets: ṯāʾ=500, etc.

The Abjad order of the Arabic alphabet has two slightly different variants. The Abjad order is not a simple historical continuation of the earlier north Semitic alphabetic order, since it has a position corresponding to the Aramaic letter samekh / semkat ס, yet no letter of the Arabic alphabet historically derives from that letter. Loss of samekh was compensated for by the split of shin ש into two independent Arabic letters, ش (shīn) and ﺱ (sīn), which moved up to take the place of samekh.

...
Wikipedia