leaving the square base

the diagonal as starting point

As in the late romanesque church of Sainte-Marie-Madeleine in Vézelay it was common in gotic style to construct adjacent groined vaults to cover the nave. But when starting from the diagonal instead of the barrel vaults one could leave the square base of a groined vault, narrowing the lateral side of them. Drag in the applet the red point to the right.

narrow bays

The nave of the church is no longer roofed by one large barrel vault, but it is divided in a serie of adjacent narrow bays, clearly marked by vertical colums and roofed by groined vaults.