The alfiz (meaning ‘a container) is an architectonic adornment, consisting of a moulding, usually a rectangular panel, which encloses the outward side of an arch. It is an Islamic Arab architectonic ornament, but has appeared in non-Arab Spanish architecture since the 8th century.

It is frequent in the Islamic Hispanic art and mozarabic art (usually in connection with the horseshoe arch). As the image illustrates, there are two alfiz variants:

Description
It is a rectangular box that can consist of a frame, a molding, a reinforcement of the wall or even large decorative panels, which enclose the outer edges of an arch.

It is an architectural ornament of Islamic architecture. It has developed particularly in the Umayyad architecture of al-Andalus, where it is often associated with the horseshoe arch, but it is also widely diffused elsewhere in the Muslim world, to adorn various types of arch.

It is also found in the Christian architecture of the Iberian Peninsula : in the Mozarabic architecture then in the Mudejar architecture.

A Alfiz starting from the impost.
B Alfiz starting from the floor.

The space between the arch and the alfiz is called enjuta or arrabá, usually richly decorated (iron-gray in the illustration). Each curved triangle is called albanega (spandrel).

The space between the arch and the alfiz is called enjuta or arrabá, and is often richly decorated (in gray in the illustrations). Each curved triangular residual surface is called albanega (angular).

The alfiz derives from the ancient Roman framings, which are found in particular on the triumphal arches. The oldest examples of this shot meet in Etruscan architecture. An alfiz can be found framing a horseshoe arch in the Visigothic art in Spain, prior to the arrival of Islam, in the church of Santa Maria de Quintanilla de las Viñas, in Mambrillas de Lara ( Province of Burgos, Castile and León ).

Source From Wikipedia