The constructive principle involved is, of course, that the slightly spreading capital presents no projections that are not abundantly strong for the weight with which they are charged, while those of the more spreading form would be more or less weak—in 
FIG. 115.appearance, if not in reality—if the weight of their charge were not supported by a thick abacus. It must be admitted that this principle does not always strictly hold, for in the Cathedral of Senlis the spreading capital of the sanctuary has an abacus which, though thick, is not much if at all thicker in proportion than those of the less spreading capitals of the choir and nave. Senlis is, however, a very early building in which the principles of the Gothic system were not yet carried out in every detail.
In Gothic architecture the abacus and the bell are usually wrought out of one stone, and the astragal is always worked upon the capital itself instead of being worked upon the shaft according to the ancient mode. The profile of the capital thus includes both of these members.
The abacus was usually square in plan until nearly the middle of the thirteenth century, except in the case of compound capitals—like those of the great western piers of the Cathedral of Paris—where the centre portion is round. The plan of the abacus is, of course, determined by the form of the load which it has to carry. As the arch sections were, for the most part, square during the early Gothic period, the plan of the abacus was also square. But when, in the more advanced Gothic, the mouldings of the archivolt give a polygonal section, the plan of the abacus assumes a polygonal form in conformity with it, as in the
