The tau cross is a T-shaped cross, sometimes with all three ends of the cross expanded. It is called a “tau cross” because it is shaped like the Greek letter tau, which in its upper-case form has the same appearance as Latin letter T.
The Greek letter tau was used as a numeral for 300. The Epistle of Barnabas (late first century or early second) gives an allegorical interpretation of the number 318 (in Greek numerals τιη’) in the text of Book of Genesis 14:14 as intimating the crucifixion of Jesus by viewing the numerals ιη’ (18) as the initial letters of Ἰησοῦς, Iēsus, and the numeral τ’ (300) as a prefiguration of the cross: "What, then, was the knowledge given to him in this? Learn the eighteen first, and then the three hundred. The ten and the eight are thus denoted—Ten by Ι, and Eight by Η. You have [the initials of the name of] Jesus. And because the cross was to express the grace [of our redemption] by the letter Τ, he says also, 'Three Hundred'. He signifies, therefore, Jesus by two letters, and the cross by one."
Clement of Alexandria (c. 150 – c. 215) gives the same interpretation of the number τιη’ (318), referring to the cross of Christ with the expression "the Lord's sign": "They say, then, that the character representing 300 is, as to shape, the type of the Lord’s sign, and that the Iota and the Eta indicate the Saviour’s name."
Tertullian (c. 155 – c. 240) remarks that the Greek letter τ and the Latin letter T have the same shape as the execution cross: "Ipsa est enim littera Graecorum Tau, nostra autem T, species crucis".
In the Trial of the Court of the Vowels of non-Christian Lucian (125 – after 180), the Greek letter Sigma (Σ) accuses the letter Tau (Τ) of having provided tyrants with the model for the wooden instrument with which to crucify people and demands that Tau be executed on his own shape: "It was his body that tyrants took for a model, his shape that they imitated, when they set up the erections on which men are crucified. Σταυρός the vile engine is called, and it derives its vile name from him. Now, with all these crimes upon him, does he not deserve death, nay, many deaths? For my part I know none bad enough but that supplied by his own shape—that shape which he gave to the gibbet named σταυρός after him by men"
The Greek word σταυρός, which in the New Testament refers to the structure on which Jesus died, appears as early as AD 200 in two papyri, Papyrus 66 and Papyrus 75 in a form that includes the use of a cross-like combination of the letters tau and rho. This tau-rho symbol, the staurogram, appears also in Papyrus 45 (dated 250), again in relation to the crucifixion of Jesus. In 2006 Larry Hurtado noted that the Early Christians probably saw in the staurogram a depiction of Jesus on the cross, with the cross represented (as elsewhere) by the tau and the head by the loop of the rho, as had already been suggested by Robin Jensen, Kurt Aland and Erika Dinkler. In 2008 David L. Balch agreed, adding more papyri containing the staurogram (Papyrus 46, Papyrus 80 and Papyrus 91) and stating: "The staurogram constitutes a Christian artistic emphasis on the cross within the earliest textual tradition", and "in one of the earliest Christian artifacts we have, text and art are combined to emphasize 'Christus crucifixus'". In 2015, Dieter T. Roth found the staurogram in further papyri and in parts of the aforementioned papyri that had escaped the notice of earlier scholars.
In the view of Tertullian and of Origen (184/185 – 253/254)) the passage in Ezekiel 9:4 in which an angel וְהִתְוִיתָ תָּו עַל־מִצְחֹות הָאֲנָשִׁים, "set a mark [tav; after the cross-shaped Phoenician and early Hebrew letter] on the forehead of the men" who are saved was a prediction of the Early Christian custom of repeatedly tracing on their own foreheads the sign of the cross.
The Hospital Brothers of St. Anthony, known as the Antonines, were a Catholic religious order of the Latin Church founded at the end of the 11th century. They wore a black religious habit marked with a blue tau. This habit became associated with their patron saint, Anthony of Egypt, who accordingly was represented as bearing on his cloak a cross in the form of a tau.
Through its association with the Antonines, this cross became known as Saint Anthony's cross, as the disease of ergotism, to whose treatment the Antonines devoted themselves more particularly, became known as Saint Anthony's fire.
In about 1500 the Antonines still had 370 hospitals, but with the identification of the fungus that caused ergotism, the reduction of epidemics and the competition from the hospitals of the (generally known as the Knights of Malta) their numbers decreased. Their last house in Europe was finally closed in 1803.
Another explanation proposed for the association of the tau cross with Saint Anthony is that the tau cross a stylized representation of the saint's staff, topped by a horizontal bar, on which he supported himself when old. A staff of that kind is represented in his hand in a painting by the German Renaissance painter Matthias Grünewald (c. 1470 – 1528) on an outer panel of his Isenheim Altarpiece. In this painting the saint remains tranquil in spite of being threatened by a fearsome demon depicted as breaking the panes of the window behind him. The saint is not shown as wearing the emblem of the Antonines.
The Antonines survive in the Middle East, especially in Lebanon, as a Maronite Church order with 21 monasteries and many schools and seminaries. They still use the bright blue tau cross on their black habits.
With the disappearance in the Western Church of the Hospital Brothers of Saint Anthony, the tau cross is now most commonly associated with the Franciscan Order and its founder, Saint Francis of Assisi, who adopted it as his personal sign after hearing Pope Innocent III talk about the Tau symbol. It is now used as a symbol of the Secular Franciscan Order.
In discussions of Roman execution crosses the tau cross is called the crux commissa. This term was invented by Justus Lipsius (1547–1606), who used it to distinguish this T-shaped cross from the now more familiar †-shaped cross and the saltire ☓-shaped cross.