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 8th century Arabic numerals. In modern Arabic, the word ʾabjadiyyah means "alphabet" in general.
In the Abjad system, the first letter of the Arabic alphabet, ʾalif, is used to represent 1; the second letter, bāʾ, is used to represent 2, etc. Individual letters also represent 10's and 100's: yāʾ for 10, kāf for 20, qāf for 100, etc.
The word "abjad" (أبجد ʾabjad) itself derives from the first four letters in the Phoenician alphabet, Aramaic alphabet, Hebrew Alphabet, etc. 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 sameḵ/semkat ס, yet no letter of the Arabic alphabet historically derives from that letter. Loss of sameḵ was compensated for by the split of šin ש into two independent Arabic letters, ش (šīn) and ﺱ (sīn) which moved up to take the place of sameḵ.