A History of the Tau Cross
The Tau Cross, in the shape of the letter T, resembles the last letter of the Hebrew alphabet as well as a Greek letter called tau. The Tau cross, although it goes by a few other names, is best known as the Franciscan Cross.
The letter tau or T is the sign which the angel gave to the Prophet Ezekiel to mark the foreheads of those Jews who were to be saved when Jerusalem would be destroyed. "Go through the whole city of Jerusalem and put a mark on the forehead of everyone who is distressed and troubled because of all the disgusting things being done in the city." (Ezekiel 9:4)
The tau also resembles the outstretched hands of Moses as he stood over the Israelites to encourage them as they fought with and overcame their Amalekite enemies (Exodus 17:11)
In Revelation 7, an angel prevents his fellow angels from destroying humanity until those who believe in God are marked on their foreheads with the tau and spared. (Revelation 7:3) The tau also adorns the foreheads of all who live in the City of God (Revelation 22:4). Knowing this, St. Anthony of Egypt, the desert father, drew the tau on his cloak.
Imitating St. Anthony, a male religious community called the Antonians painted a large tau on their habits. Founded in 1095, their ministry was caring for lepers, and they had houses for lepers around Assisi. St. Francis must have known these brothers.
Pope Innocent III, the pontiff who approved St. Francis’ Rule of Life, spoke at the Fourth Lateran Council about the importance of the tau. He also incorporated it onto his own crest and ended his message with "Be champions of the tau."
Is it any wonder that, following the Council which he attended, Saint Francis shaped his habit in the form of a tau and also drew a tau on it while using a tau for his signature. He has had it painted on doors and walls and would trace the tau over himself before taking any action.
--Angelo Erasmus Stagnaro (Adapted from an original article at http://www.ncregister.com/blog/astagnaro/the-tao-of-the-tau )