A symbolizes Christ as principle of all because it is the first letter of greek alphabet. "I am the alpha" says the Apocalypse. The meaning of A is explained in the inscription on the Chrismon’s ring of Jaca Cathedral who says, "In this sculpture you will recognize, reader, the following: P designates the Father, A the begotten and the double (letter) the Begetter Spirit, the three are really one and the same Lord".
Catedral de Jaca (Huesca), Santa María de Huerta (Soria), Santa María de la Oliva (Navarra), Santa María de Gradefes (León), Santa María de Veruela (Zaragoza), Santiago de Agüero (Huesca), Santes Creus (Tarragona), Santa María de Sirga (Palencia), San Gil de Luna (Zaragoza).
Read more in The stonemason’s signs in the Romanesque churches in Spain.