In the earlier stage of rib vaulting, the arched ribs consisted of independent or separate voussoirs down to the springing; the difficulty, however, of working the ribs separately led to two other important changes : ( 1 ) the lower part of the transverse diagonal and wall ribs were all worked out of one stone; and ( 2 ) the lower horizontal, constituting what is known as the tas-de-charge or solid clerestories, the wall rib was stilted, and this caused a peculiar twisting of the web, where the springing of the wall rib is at K : to these twisted surfaces the term ploughshare vaulting is given.
