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 the Latin letter T.

Another name for the same object is Saint Anthony's cross or Saint Anthony cross, a name given to it because of its association with Saint Anthony of Egypt.

It is also called a crux commissa, one of the four basic types of iconographic representations of the cross.

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". (translated: "For the Greek letter Tau itself, or our T, is the shape of the cross".)

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.