west end is the Great West Window, known as the 'Heart of Yorkshire', second-largest among the church's 128 windows. This window was designed and built along with the rest of the west front by the master mason Ivo de Raghton in 1338–39. The tracery is in the Flamboyant or Curvilinear Decorated style of English Gothic architecture. Because of deterioration of stone mullions, the tracery was replaced in the late 1980s with an exact copy. The east end of the Minster was built between 1361 and 1405 in the Perpendicular Gothic style. Despite the change in style, noticeable in details such as the tracery and capitals, the eastern arm preserves the pattern of the nave. The east end contains a four-bay choir; a second set of transepts, projecting only above half-height; and the Lady Chapel. The transepts are in line with the high altar and serve to throw light onto it. Behind the high altar is the Great East Window, the largest expanse of medieval stained glass in the country, which underwent a decade-long restoration and conservation project, completed in 2018. The sparsely decorated Central Tower was built between 1407 and 1472 and is also in the Perpendicular style. Below this, separating the choir from the crossing and nave is the striking 15th-century choir screen. It contains sculptures of the kings of England from William the Conqueror to Henry VI with stone and gilded canopies set against a red background. Above the screen is the organ, which dates from 1832. The West Towers, in contrast with the Central Tower, are heavily decorated and are topped with battlements and eight pinnacles each, again in the Perpendicular style. In 2003, English Heritage made publicly available a monograph on the architectural history of York Minster. The book charts the construction and development of the minster based on the architectural recording of the building from the 1970s. Stained glass Some of the stained glass in York Minster dates back to the 12th century and much of the glass (white or coloured) came from Germany. The glass was painted, fired, then joined with lead strips (came) into the windows. The tall and wide Great East Window was created by John Thornton in the early 15th century; he was paid £66 for the work. This is the largest expanse of medieval stained glass in the country, according to the Minster. The window represents scenes from the Book