The Arabic script has never been one hand. The earliest Quranic manuscripts are written in the angular styles collected under the name Kufic, letters broad and low with long horizontals, a script laid out for the page as a designed object rather than for speed. Kufic sat comfortably on parchment in the horizontal format of the early codices, and it moved without difficulty onto stone, brick, tile, coin and textile, where its geometry could be stretched, squared and woven into architectural ornament long after it had ceased to be used for books.
What displaced it was the cursive family. From the Abbasid period a set of rounded hands took over the copying of books and documents, and by the tenth century the tradition credits the vizier Ibn Muqla with fixing their proportions against a fixed measure: the dot made by the pen nib, the height of the alif counted in dots, and a circle whose diameter is that height. Ibn al-Bawwab and Yaqut al-Mustasimi are named after him as the writers who refined the system. Out of it came the six pens of classical calligraphy, among them naskh, the clear book hand in which most Qurans have been copied since, and thuluth, the large display hand of inscriptions and titles.
Regional traditions then diverged. The Maghreb kept a rounded hand of its own with distinctive pointing for fa and qaf and its own order for the abjad numerals. Iran developed nastaliq, hung and sloping, for Persian poetry. The Ottoman lands carried naskh and thuluth to a standard still taught as authoritative. Because Islam gave the script the central place that figural painting held elsewhere, these hands are not merely styles of writing but the principal sacred art of a civilisation.