The Arabic Abjad system continues at this point with letters not found in other alphabets: thāʾ = 500, khāʾ = 600, dhāl = 700, etc.
Abjad numerals in Arabic are similar to the earlier alphanumeric codes of Hebrew gematria and Greek isopsephy.
The Mashriqi abjad sequence, read from right to left, is: This is commonly vocalized as follows: Another vocalization is: In the Maghrebian abjad sequence (quoted in apparently earliest authorities and considered older[2]), loss of samekh was compensated for by the split of tsade צ into two independent Arabic letters, ض (ḍad) and ص (ṣad), which moved up to take the place of samekh.
which can be vocalized as: Another vocalization is: In Abu Muhammad al-Hasan al-Hamdani's encyclopædia Kitāb al-Iklīl min akhbār al-Yaman wa-ansāb Ḥimyar (کتاب الإكليل من أخبار اليمن وأنساب حمير), the letter sequence is:[4] Modern dictionaries and other reference books use the newer hijāʾī (هجائي) / alifbāʾī (أَلِفْبَائِي) order, which partially groups letters together by similarity of shape, and is never used as numerals: In the Maghrebian hijāʾī / alifbāʾī order (replaced by the Mashriqi order[3][unreliable source?
", and "C." (or, rarer, Roman numerals: I, II, III, IV), in Arabic, thus "أ", then "ب", then "ج", not the first three letters of the modern hijāʼī order.
Like the Abjad order, it is used in modern times for numbering outlines and points of information, including the first six days of the week.