[...] The carving of boss stone requires two important data, first of all, the horizontal projection of the boss stone, we can obtain it from the full scale design of the vault’s plan. This design is redrawn on a horizontal flat face of a piece of stone and allows us to start carving vertically, taking out the leftover stone (photos 10,11,12).

The second part in the carving of a boss stone is the most complex task and gives us a precise idea of the degree of accuracy reached by the medieval stone mason. It is the cutting, with its correct slope, of the different arches that fit in the boss stone. To accomplish this task it’s necessary to work whit the elevation of the vault. Drawing in its right position, in plan and elevation, each of these pieces (photo 13).

As the picture shows only then is it possible to determine the angles of the different ribs witch attain a boss stone, In this picture we can see how the right angle and the left one of a boss stone are determined by tracing two right lines from the centre of the arch both sides of the piece. These angles could later allow us to carve the arms of the boss stone with its right slope (photo 14). [....]

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