The image shown here is of a window of the Cathédrale royale de Saint-Denis, (XII century) near Paris, where the angel of God marks the foreheads of all those who are saved with the TAU. This episode is related by the prophet Ezekiel in Chapter 9, Verse 4 of the Old Testament. The symbology of the TAU was particularly developed during the late Middle Ages, when it was represented in several churches.