This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Arabic Letters gathers the twenty eight letters of the Arabic script the atlas records, twenty two inherited from the Northwest Semitic abjad and six added later, and each carries its own position, sound, joining behaviour and numeral value.
The abjad reckoning gives every letter a numeral value in an older order that differs from the everyday modern alphabetical order; Alif carries one, and the six added letters, Tha, Dhal, Kha, Dad, Za and Ghayn, without inherited Northwest Semitic names, were appended at the end of the abjad order and so carry its highest values, running from six hundred up to Ghayn's one thousand, the highest of all. Most letters descend directly from a Northwest Semitic ancestor and keep its numeral value: Fa holds the eighty that Hebrew pe holds, Lam the thirty of Hebrew lamed, Nun the fifty of Hebrew nun.
Every letter also carries a joining behaviour: most connect on both sides, but six, among them Alif, Dal, Ra and Waw, join only to the right, breaking a word into separate connected groups wherever they appear. The tradition further sorts the letters into sun and moon letters, which governs how the definite article's lam is pronounced before them, sun letters assimilating it in speech, moon letters keeping it distinct and pronounced. Several letters, among them Alif, Ha, Lam, Mim and Nun, stand among the disjoined letters at the head of certain suras of the Quran, isolated single letters or short combinations whose sense the tradition itself does not claim to have settled.