The key symbolizes the divine power delegated into the pope. The key is the symbol of St. Peter, the first Pope, whom Christ delivered the power of binding and loosing.
The crosier symbolizes the religious power of abbot. It is a symbol of shepherd. The abbots of Cluny and Citeaux and the bishops use the crosier as a symbol of their function.
The royal crown symbolizes earthly power. It is the symbol representing the king in the medieval chess game.
Santa María de Moreruela (Zamora), Santiago de Agüero (Huesca), Santa María de Gradefes (León), Santa María de Moreruela (Zamora), Catedral de Santiago de Compostela, Santa María de Albocaça (Portugal), Santo Sepulcro (Zamora), Santa María de Eunate (Navarra), Catedral de Santiago de Compostela.
Read more in The stonemason’s signs in the Romanesque churches in Spain.