A pier of masonry, sometimes standing alone, sometimes thrown out from the aisle wall opposite the point to be propped, formed the solid part of this buttress; it was carried to the requisite height and a flying arch spanning the whole width of the aisles was thrown across from it to the wall at the point whence the vault sprung. The pier itself was in many cases loaded by an enormous pinnacle, so that its weight might combine with the pressure transmitted along the slope of the flying arch to give a resultant which should fall within the base of the buttress. The back of such an arch was generally used as a water channel. The forest of flying buttresses round many French cathedrals produces an almost bewildering effect, as, for instance, at the east end of Notre Dame;--our English specimens, at Westminster Abbey for example, are comparatively simple. _Towers._ The gable and the tower are developments of the walls of the building. Gothic is _par excellence_ the style of towers. Many towers were built detached from all other buildings, but no great Gothic building is complete without one main tower and some subordinate ones. In the E. E. style church towers were often crowned by low spires, becoming more lofty as the style advanced. In the Dec. style lofty spires were almost universal. In the Perp. the tower rarely has a visible roof.[12] The artistic value of towers in giving unity coupled with variety to a group of buildings can hardly be exaggerated. The positions which towers occupy are various. They produce the greatest effect when central, _i.e._ placed over the crossing of the nave and transepts. Lichfield, Chichester, and Salisbury may be referred to as examples of cathedrals with towers in this position and surmounted by spires. Canterbury, York, Lincoln, and Gloucester are specimens of the effectiveness of the tower similarly placed, but without a spire (Fig. 12). At Wells a fine central octagon occupies the crossing, and is remarkable for the skill with which it is fitted to the nave and aisles internally. Next to central towers rank a pair of towers at the western end of the building. These exist at Lichfield with their spires; they exist (square-topped) at Lincoln, and (though carried up since the Gothic period) at Westminster.[13] Many churches have a single tower in this position (Fig. 13). The obvious purpose of a tower, beyond its serviceableness as a feature of the building and as a landmark, is to lift up a belfry high into the air: accordingly, almost without exception, church and cathedral towers are designed with a large upper story, pierced by openings of great size and height called the belfry stage; and the whole artistic treatment of the tower is subordinate to this feature. It is also very often the case that a turret to contain a spiral staircase which may afford the means of access to the upper part of the tower, forms a prominent feature of its whole height, especially in the Dec. and Perp. periods.