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 and English 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’ 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"


Example of a staurogram in the Catacombs
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.

Catacomb stayros
Tau /ˈtɔː, ˈtaʊ/ (uppercase Τ, lowercase τ; Greek: ταυ [taf]) is the
19th letter of the Greek alphabet. In the system of Greek numerals
it has a value of 300.

The name in English is pronounced /taʊ/ or /tɔː/, but in modern
Greek it is [taf]. This is because the pronunciation of the
combination of Greek letters αυ has changed from ancient to
modern times from one of [au] to either [av] or [af], depending
on what follows (see Greek orthography).

Tau was derived from the Phoenician letter taw Phoenician
taw.svg  Letters that arose from tau include Roman T and Cyrillic
Te (Т, т).

The letter occupies the Unicode slots U+03C4 (lowercase) and
U+03A4 (uppercase). In HTML, they can be produced with named
entities (τ and Τ), decimal references
(τ and Τ), or hexadecimal references (τ
and Τ).
In ancient times, tau was used as a symbol for life or resurrection,
whereas the eighth letter of the Greek alphabet, theta, was
considered the symbol of death.

In Biblical times, the taw was put on men to distinguish those who
lamented sin, although newer versions of the Bible have replaced
the ancient term taw with mark (Ezekiel 9:4) or signature (Job 31:35).
Its original sound value is a voiceless alveolar plosive, IPA /t/
The symbolism of the cross was connected not only to the letter
chi but also to tau, the equivalent of the last letter in the Phoenician
and Old Hebrew alphabets, and which was originally cruciform in
shape; see Cross of Tau
Christ on a TAU Cross
Kreuzigung by Konrad Witz

Source: The Yorck Project (2002) 10.000 Meisterwerke der Malerei (DVD-ROM), distributed by
DIRECTMEDIA Publishing GmbH. ISBN: 3936122202

Public domain        
This work is in the public domain in its country of origin and other countries and areas where the copyright term is the
author's life plus 100 years

The work of art depicted in this image and the reproduction thereof are in the public domain worldwide

Copyright EMMI TOSF Ministry.
The Tau’s resemblance to the cross, this sign was very dear to St. Francis of Assisi, so
ancient prophetic sign was actualized, regained its saving power and expressed the
beatitude of poverty, which is an essential element of the Franciscan way of life