Look closely at the walls of many medieval churches and, if the light is right, carefully inscribed marks can be seen. These are masons’ marks and they were made by the stonemasons who cut the blocks that make up the walls, piers, arches and windows of the churches.
The stonemason, after he had cut the stone, would inscribe his own individual mark. This would prove that he cut that piece of stone and thus get paid for his work. Usually the master stonemason, would then put his individual mark on the stone to confirm that that piece of stone was ready to go into the building that is being built.
Normally the side of the stone with the masons’ marks on was inserted inwards into the building, so it can’t be seen, but sometimes that wasn’t always the case and you can see some on churches around London today.