"Go through the city of Jerusalem and put a TAU on the foreheads of those who grieve and lament over all the detestable things that are done in it." (Ezekiel 9:4)

The Tau, the last letter of the Hebrew alphabet, was very well known at the time of St. Francis, and was already used by the Semites, Greeks and Latins.

In St. Francis' time, devotion to the Tau was accompanied by devotion to the name of Jesus, which was incessantly repeated, and was also understood to mean conversion.

On November 11, 1215, Pope Innocent III opened the Fourth Lateran Council with an invocation to this symbol already present in the Old Testament, which for Christians was related directly to the cross and salvation.  Pope Innocent III stated, "The TAU has exactly the same form as the Cross on which our Lord was crucified on Calvary.  Only those marked with this sign, who have mortified their flesh and conformed their life to that of the Crucified Savior, will obtain mercy."

 Upon hearing this, St. Francis adopted the TAU as his symbol, to express his devotion to the cross and to the whole person and mission of Christ.

The Tau can be seen in Assisi, on the door of the Pilgrims' Oratory or in various places inside the basilica named after the saint, as well as above cells and other areas of Franciscan monasteries.