The Saint Andrew’s Cross, X, symbolizes Christ being the first greek letter of his name Xρίστός. It is the second person of the Trinity. Xρίστός means the Anointed. It is in the origins of the Chrismon. Variants of X are done closing one of the open sides forming a triangle, San Marcos de Leon, or closing the two sides forming two triangles, Santa Maria de Monsalud. Thus with one or two ends closed the sign has two meanings: Christ by the Saint Andrew’s Cross and the Trinity by the triangle.
Santa María del Templo (Villalba de los Alcores, Valladolid), Catedral vieja de Salamanca, Calatrava la Nueva (Ciudad Real), San Miguel de Fuentidueña (Segovia).
Read more in The stonemason’s signs in the Romanesque churches in Spain.