by his son Thomas. One of the original decorative features was a kind of blind tracery; blank vertical panels with cusped, or angular tops in the interior; and, on the exterior, thin stone mullions or ribs extending downward below the windows creating perpendicular spaces. These became the most characteristic feature of the style. The earliest Perpendicular in a major church is the choir of Gloucester Cathedral (1337–50) constructed when the south transept and choir of the then Benedictine abbey church (Gloucester was not a bishopric until after the Dissolution of the Monasteries) were rebuilt in 1331–1350. It was likely the work of one of the royal architects, either William de Ramsey, who had worked on the London cathedral chapter house, or Thomas of Canterbury, who was architect to the king when the transept of Gloucester Cathedral was begun. The architect preserved the original 11th-century walls, covering them with Flamboyant mullions and panels. The east window of Gloucester choir has a Tudor arch, filling the wall with glass. The window tracery matches the tracery on the walls. The Henry VII Chapel at Westminster Abbey is a major example of the late Perpendicular style, with its walls of glass and elaborate fan vaults. Another important example is St George's Chapel at Windsor Castle, begun in 1475. The vault of the chapel was contracted to the master-mason John Aylmer in 1506. Characteristics Towers were exceptionally tall, and frequently had battlements. Spires were less frequent than in earlier periods. Buttresses were often placed at the corners of the tower, the best position for providing maximum support. Notable Perpendicular towers include those of York Minster and Gloucester Cathedral, and the churches of Boston, Warwickshire and Wrexham, Taunton. Stained glass windows were so large that the walls between were reduced to little more than piers. Horizontal mullions, called "transoms", often had to be added to the windows to give them greater stability. Tracery was a major feature of decoration. In the larger churches, the entire surface from ground to summit, including the battlements, was covered with panels of tracery composed of thin stone mullions. It also appeared frequently in the interior, and often carried the designs in the window tracery down to the floor. Roofs were frequently made of lead, and usually had a gentle slope, to make them easier for walking. The roof timbers on the interior were often exposed to view from below, and