The Basics Of Islamic Calligraphy:
The legibility of a text and the beauty of its line requires rules of proportion. According to Arab calligraphers , the proportions of the characters always remain in a constant relationship : they all refer back to the size of "alif" the first letter of the alphabet."Alif" is taken as the module of every Arabic calligraphic system, this choice and method are due to Islamic religious and mystical concepts, the length of the alif varies according to the writer , but all agree on using the dote as the universal unit of proportion. This is a square impression formed by pressing the tip of the pen onto the paper. The dimentions of each side of this square dot thus depend on the way in which the pen has been cut,and on the pressure exerted by the fingers. This pressure had to be sufficiently delicate and precise to separate the two sides of the nib.
What is meant by an Arabic dot and its measurement? The history of units of measurement is a very difficult subject, for these units change their names and designations from region to region,and thus make systematization impossible. Al-Qalqashandi has shown that the standard pen was the official type used by the Abbasi Caliphas(rulers of the Islamic Empire) for signatures or initials. This pen ,known as "Tomar", consisted of twenty-four hairs of a donkey(or the equivalent), properly arranged and set.

Each calligrapher cut his pen in accordance with his owen usage and that of his native land, and also in accordance with the kind of text he was to transcribe. In this sense,Islamic callegraphy styles of script are definable by the pen and the width of the nib . In the thuluth style the width is equal to one-third of the Tomar, and therefore the dot used for thuluth is also one- third.
The height of an alif varies from three to twelve dots depending on the author and the style of the script,its width then being equivalent to one dot .the important thing was to establish the height for each text. once the Arabic calligrapher had his alif module, he would draw it in the same way throughout the text . this was the general geometric principle, although in practice the calligrapher introduced variations , the arrangement of this variations is of grate interest.
Alif was also used to measure the diameter of an imaginary circle within which all Arabic letters could be written. The calligrapher composed the characters, and therefore proportioned them according to the dot width, the size of the alif, and the diameter of the circle. These three elements where chosen by him.
(from "the splendor of Islamic calligraphy"/by Abdelkebir Khatibi and Muhammed Sijelmassi.)