Thuluth (Arabic: ثلث "one-third", Turkish: Sülüs) is a script variety of Islamic calligraphy, which made its first appearance in the fourth century of the Hegira (11th century CE). The straight angular forms of Kufic were replaced in the new script by curved and oblique lines. In Thuluth, one-third of each letter slopes, from which the name (meaning "a third" in Arabic) comes. It is a large and elegant, cursive script, used in medieval times on mosque decorations. Various calligraphic styles evolved from Thuluth through slight changes of form.
The best known artist to write the Thuluth script at its zenith is said to be Mustafa Rakım Efendi, -a painter- that set the barrier in Ottoman Calligraphy, a barrier that many said has still not been crossed until this day.
Thuluth was used to write the headings of surahs, Qur'anic chapters. Some of the oldest copies of the Qur'an were written in Thuluth. Later copies were written in a combination of Thuluth and either Naskhi or Muhakkak, while after the 15th century Naskhi came to be used exclusively.
An important aspect of Thuluth script is the use of Harakat ("Hareke" in Turkish) for representing vowel sounds, and other marks that were created to beautify the script. The grammatical Harakat follows the normal rules for any Arabic script, but the 'Art' markings follow their own rules in relation to placement and grouping. One common grouping technique is to separate the marks written below letters from those written above. The stylistic rules allow for great creativity as to shape and orientation.