A keystone placed in the centre of a vault was held there by a combination of great strength, as it became a point of abutment for all the main ribs, whose thrust was distributed against four piers and hence exteriorly by buttresses to the ground. A good stone, therefore, in this position could have extraordinary dimensions, and was susceptible of a variety of treatment. In some French examples it was extended, or rather hung, considerably below the surface of the vault and ornamentally carved, while in England, in the late so-called Perpendicular Gothic, it formed the centre of a large pendant, or circular hanging ornament, which in some cases came down almost to the level of the springing of the ribs. This construction was used chiefly in connection with the fan-vaulting, in which English architects excelled, which may indeed be said to be an English invention and monopoly, as no examples of it are found elsewhere. The name explains, in measure, the form taken by the ribs, which, spreading out from the sheaf of mouldings in the pier, trace a perfect semicircle on the upper ceiling, their intervening spaces being occupied by panels. The four semicircles thus traced by the ribs, starting from four piers of a compartment, are each tangent to a central and whole circle forming the contour of the pendant. To be successful this requires that the compartment or space included between four piers, two on each side of the nave or choir, should be a square, otherwise the circles do not touch, and the lines are inharmonious. The chapels of Henry the Seventh, at Westminster, and of St. George, at Windsor, contain the best examples of fan-vaulting, and are very beautiful in general effect, though it is questionable whether such constructive tricks are worthy of unrestricted praise, while the abuse of panelling in which English architects indulged in these later Gothic buildings, by which the whole wall and ceiling surface was cut up in an unending repetition, was certainly blameworthy, and tended to reduce their art to a mechanical science. They excelled, however, in all mechanical workmanship, in which perhaps that employed in the execution of timber roofs is the most remarkable. These were in a measure, at least upon so large a scale, a feature wholly English, for nothing approaching them is found elsewhere. The roof of Westminster Hall is the most justly celebrated and is unique in general character. The natural stonework showing all its joints was generally left untouched in the interior of Gothic buildings, and afforded the best finish as well as contrast to the stained glass in the windows.