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The basic function of the stonemason’s signs is to protect the temple of the devil and all the evil spirits that try to penetrate it to confuse the souls in the battle between God and the Devil. For that reason the basic signs arise from the Chrismon, Pater (P), Christ (+) and (X), Spiritus Sanctus (S), Alpha (A), Omega (ω); others arise by having great sacred meaning, Trinitas (T), Mary, Crosier; besides of the stonemason tools with which God has built the temple, Square L, Compass (˄) and Peak.
The signs were not made to be seen but to protect the temple.
Read more in The stonemason’s signs in the Romanesque churches in Spain.