A middle-sized muqarnas (stalactite) niche, on the right side of the main entrance to the Yeşil Cami. The style is still close to its late-Seljuk examples. The elegant arabesques and floral motifs of the panels on the left and the right of the muqarnas vaulting announce the later Classical Ottoman style.
Above the niche: several calligraphic inscriptions in elegant sülüs script.

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.