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."
Thomas of 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."
In the famous blessing of Brother Leo, Francis wrote on parchment, "May the Lord bless you and keep you! May the Lord show His face to you and be merciful to you! May the Lord lift up His countenance upon you and give you peace! God bless you Brother Leo!" Francis sketched a head (of Brother Leo) and then drew the TAU over this portrait.
Due, no doubt, in large part to Francis' own affection for and devotion to the TAU, it has been a well recognized and accepted Franciscan symbol among Franciscans of various denominations and of all orders within those denominations for centuries. It remains so today. The TAU carries with it all of the symbolism of the Cross of Christ as well as Francis' ideal of life and dream for himself and his followers.
Br. JR, OSF :)