In the apse of the Church of Horta de Sant Joan (1316), there are a large mason marks squads. The objective is to determine the extent and proportionality of this instrument and its possible use in the medieval factory. The approach is to determine the performance of the squad rotation, especially in staking pentagonal apse. The squads are used as an abacus for constructing regular polygons. These instruments, by small adjustments and calibrations, allow very effective results for the construction of polygons 11, 13, 14, 17 or 19 sides, that do not appear in mathematicis sources, or in the few codices Practica Geometriae.