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The triangle is the most genuine representation of the Trinity: Father, Son and Holy Spirit. Three Persons in One God. Three sides in a single figure. The triangle represents God in religious iconography. In occasions with the eye of God that sees everything. It is the greek letter "delta". It is the beginning of the pronunciation of the genitive of the Greek name of God Διός (Diós).
Churches:
Nuestra Señora de la Gloria de Casbas (Navarra), Catedral de Plasencia, Santa María de Olite (Navarra), Santes Creus (Tarragona), San Martín de Frómista (Palencia), Nuestra Señora de Veruela (Zaragoza).
Read more in The stonemason’s signs in the Romanesque churches in Spain.