On the picture: The muqarnas (stalactite) vaulting of the monumental entrance, in a style still close to its late-Seljuk examples. Beneath them: the long and large ‘kitabe’ (building inscription), mentioning sultan ‘Mehmed bin Bâyezid bin Murad bin Orhan’ (father, grandfather, over-grandfather) and the date of 1419. The script used is ‘sülüs’.
The calligraphic text measures 82 x 342 cm (middle panel) and two times 82 x 129 cm; the small panel on the left mentions the architect Hacı Ivaz Paşa, who was also an army commander and a vizier; he is the architect of the Yeşil Türbe (mausoleum) too.

Sülüs (from Arabic: ثلث‎ ṯuluṯ "one-third"), also referred to as ‘thuluth’, is a script variety of Islamic calligraphy invented in Persia, which made its first appearance in the 11th century AD. The straight angular forms of the older Kufic script were replaced in the new script by curved and oblique lines. In Sülüs, 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.

Correspondent: J.M.Criel, Antwerpen.
Sources: ‘Vakıf Abideler ve eski Eserler’ - Vakıflar Genel Müdürlüğü III, Ankara 1983 - Wikipedia,
& ‘Bursa Yeşil Külliyesi’ (Erdem YÜCEL, Arkeolog)