in calligraphy, medieval Islāmic style of handwritten alphabet. Thuluth (Arabic: “one-third”) is written on the principle that one-third of each letter slopes. It is a large and elegant, cursive script, used in medieval times on mosque decorations. It took on some of the functions of the early Kūfic script; it was used to write surah (or sura; Qurʾānic chapter) headings, religious inscriptions, and princely titles and epigraphs. It was also used for many of the large copies of the Qurʾān produced from the 13th century.