The Tau Cross – a Franciscan cross
The TAU -rhymes with ‘now’- represents the cross and is the symbol used by all Franciscans. The first recorded reference to the TAU is from Ezekiel 9:4, "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." The TAU is the last letter of the Hebrew alphabet and looks very much like the letter "T".
At the Fourth Lateran Council, on November 11, 1215, Pope Innocent made reference to the TAU and quoted the above verse in reference to the profaning of the Holy Places by the Saracens. It is widely accepted that St. Francis was present at the Fourth Lateran Council and that he heard the words of Pope Innocent III when he said, "The TAU has exactly the same form as the Cross on which our Lord was crucified on Calvary, and only those will be marked with this sign and will obtain mercy who have mortified their flesh and conformed their life to that of the Crucified Savior. From then on, the TAU became Francis' own coat of arms.
Francis used the TAU in his writings, painted in on the walls and doors of the places where he stayed, and used it as his only signature on his writings.
St. Bonaventure said, "This TAU symbol had all the veneration and all the devotion of the saint: he spoke of it often in order to recommend it, and he traced it on himself before beginning each of his actions."
Celano, another Franciscan historian writes, "Francis preferred the Tau above all other symbols: he utilized it as his only signature for his letters, and he painted the image of it on the walls of all the places in which he stayed."
The TAU then embraces both the symbolism of the Cross of Christ as well as Francis' ideal of life and his vision for himself and his followers.
(With thanks to Ken Norian, TSSF)