ABSTRACT
In current historiography on fan vaults, the geometrical method employed to define the shape of the blocks constituting the conoidal shells in joined masonry is based on a horizontal plane opportunely translated to the extrados of the vault, as wonderfully described by Robert Willis. His conclusions came after noticing the flat surface of the extrados of the voussoirs of these vaults. It is worth looking afresh at Willis’s theory for two reasons. Firstly, because it cannot be confirmed as no English medieval drawing describing this procedure exists. Secondly, as commented by Leedy in his study on fan vaults, Willis’s conclusions were based on built examples featuring a top flat plane, which are the exception, not the norm. It is proposed in this paper that the individual stone blocks may have possibly been defined by a different method, which uses a plane tangent to the vertical ribs, as opposed to a horizontal one.
