Abjad and Vowel Marks
The oldest order of the letters is also a number system: the first letter counts as one, the tenth as ten, the nineteenth as one hundred, and the last as one thousand.
Every abjad carries two orders. The one children learn today — in Arabic, letters grouped by shape — is a late, practical arrangement for dictionaries. Underneath it lies an older sequence, the one the word 'abjad' spells out: alif, ba, jim, dal, and so on to the end of the inventory.
That old order is a counting system. The first nine letters are the units, one to nine; the next nine are the tens, ten to ninety; the next nine are the hundreds, one hundred to nine hundred; and the final letter — ghayn in the Arabic reckoning — stands for one thousand. Numbers are written by stringing letter values together.
The scheme is not a medieval invention. Abecedaries from the city of Ugarit, written in cuneiform around the thirteenth century BCE, already list the letters in essentially this sequence — which means the counting order is older than most of the scripts that use it.
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Hebrew kept the same correspondence with its 22 letters: aleph is one, yod is ten, qof is one hundred, tav is four hundred. Reading numbers out of words — and words out of numbers — became the interpretive practice known as gematria, and the same arithmetic in Arabic goes by the name abjad numerals.
For centuries the letter-numbers did everyday work: numbering folios and quires in manuscripts, ordering lists, marking years on coins and buildings. They appear wherever a writer of Arabic, Hebrew, Syriac or Greek needed numerals before the Indian digits became standard.
The modern dictionary order broke the link. Sorting Arabic letters by shape — ba, ta, tha side by side — made lookup easier and scattered the numeric sequence. The old order survives wherever the numbers do: in gematria, in traditional foliation, and in the very name of the system itself.
Further reading