On the picture: Close-up of one of the borders of the ‘mihrab’ (prayer niche, which indicates the direction of Mecca). The niche is completely executed in glazed tiles; this border shows elegant calligraphy in which two scripts were used: kufic (in yellow) and sülüs (in white).

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 & Wikipedia