On the picture: The wall above the window in the southeast wall of the mihrab area.
The border in glazed tiles shows elegant calligraphy in which two scripts were used: kufic (in yellow) and sülüs (in white). The painted calligraphic works higher on the wall date from the 19th century, when the mosque was renovated after the 1855 Bursa earthquake.

Kufic is the oldest calligraphic form of the various Arabic scripts and consists of a modified form of the old Nabataean script. Kufic developed around the end of the 7th century in Kufa (Iraq). Until about the 11th century it was the main script used to copy Kurans.

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