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:
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).
References
^Alfiz: "DRAE. Diccionario de la Lengua Española. Vigésima segunda edición", Espasa Calpe, S.A, 2003