In the 14th century, the Lorraine region was home to several renowned glassmakers, established in Verdun and Toul. Although born in Westphalia, Hermann of Münster was the most famous among them. In the Middle Ages, stained-glass windows had an active role in the religious education of Christian populations who were often illiterate. Those of Metz Cathedral never show a glimpse of the outside. People could therefore focus only on their content. The large O, as the great rose window is mentioned in the account books of the cathedral in 1385, is the masterpiece of the German artist.