The large window in the centre of the Duomo's apse - window V20 - is dedicated to the stories from the Apocalypse taken from the Book of John, and is made up of three rows of four columns each. It is also known as the "Apocalypse Window ", the "Raza Window", or the "Visconti Window": these nicknames come from the presence, in the upper part of the window, of a sun-shaped insert or "raza", marble on the outside and composed of marvellous glass inside. The creation of this decoration was probably personally commissioned by Gian Galeazzo Visconti himself at an early stage in the project, when the Duke still had hope of presenting the Cathedral as a distinctly Visconti work. The stained-glass window is divided in two parts, the upper and central section still holds 15th and 16th-century panels in bright blue, purple, and red, while the lower one dates back to the 19th century from the workshop of Giovanni Battista Bertini and his sons, Giuseppe and Pompeo.