The letter 'ת' or 'tau' (or taf or tav, ymmv) is the last letter of the Hebrew alphabet. For many Christians, the symbol is a representation of the cross of Jesus.
The Tau Cross is particularly identified with Saint Francis of Assisi and his Franciscans. Francis adopted the symbol as his own, working it into his signature. The significance of the symbol comes from the Bible verse found in Ezekiel 9:4, which reads:
And the LORD said to him, "Go through the midst of the city, through the midst of Jerusalem, and put a mark on the foreheads of the men who sigh and cry over all the abominations that are done within it." (New King James Version)
Although lost in modern English translations, the Latin Biblia Sacra Vulgata does specify that the "mark" in question is the Tau.
et dixit Dominus ad eum transi per mediam civitatem in medio Hierusalem et signa thau super frontes virorum gementium et dolentium super cunctis abominationibus quae fiunt in medio eius
At the Fourth Lateran Council in 1215, Pope Innocent III again used this image from the prophet Ezekiel for the theme of his opening homily.
"We are called to reform our lives, to stand into the presence of God as righteous people. God will know us by the sign of the tau marked on our foreheads."
The Tau became a symbol from the Council for spiritual renewal
in the Church
. It is believed that Francis himself might have been present at that Council, and was inspired to use the Tau Cross as his symbol from then on.
Today, followers of Francis, whether clergy or laity, use the Tau as an exterior sign of commitment to be models of faithfulness and compassion.