Easton's Bible Dictionary lists the forms in which such gibbets are represented:
Of these forms, certain writers, notably Jehovah's Witnesses, accept for the gibbet on which Jesus died only the meaning "a pale, a strong stake, a wooden post.
The Greek terms used in the New Testament of this gibbet are stauros (σταυρός) and xylon (ξύλον).
The Greek-English Lexicon of Liddell and Scott, the major reference work on the Greek language from Homeric to early Christian times reports that the meaning of the word "σταυρός" in the early Homeric form of Greek, possibly of the 8th to 6th century B.C., and also in the writings of the 5th-century B.C. writers Herodotus and Thucydides and the early-4th century B.C. Xenophon, is that of a stake; but that in the writings of the first-century B.C. Diodorus Siculus and in later writers, such as Plutarch and Lucian, it refers also to a cross.
The word "ξύλον" was used of wooden objects as varied as firewood, a cudgel, a wooden collar or stocks for a prisoner, a gallows, a stake, a table, a wooden spoon, and a live tree.
Much the same can be said of the Latin word crux, which, not being Greek, does not appear in the New Testament, but was used by writers from the second century onward to refer to that on which Jesus died. The most authoritative Latin-English Dictionary, that of Lewis and Short, indicates that in the literal sense, crux meant, in general, a tree, frame, or other wooden instruments of execution and, in particular, a cross" (the most common sense) or (in one instance) "the pole of a carriage". The meaning of the word varied over the centuries. The writers who hold that Jesus died on a simple upright stake appeal to a statement in the article "Cross" in McClintock & Strong’s Cyclopedia that, even when the cross with a traverse beam had become the usual form, the meaning of the crux simplex type of cross, i.e. the single torture stake, was still in use at the third century A.D.
People employed each of these words (the first two in Greek, the third in Latin) with more than one meaning. Accordingly, no conclusion can be drawn from the words in themselves. Nor is the authority of standard lexicons of Greek and Latin decisive for those who hold a different view: for instance, those who believe that Jesus died on a stake do not accept the attribution, by the standard lexicographical work of the Greek language, of the meaning "cross", rather than "stake", to the word "σταυρός" in . "The exact technical form and significance of execution are not conveyed by the words stauros and (ana)stayroo without further definition", says The New International Dictionary of New Testament Theology. And the scholars whose works the two sides quote also disagree.
It is therefore only by the descriptions they give of that gibbet that the early writers who speak of it indicate the shape that they envisaged it had.
Using the Greek word σταυρός in its verbal form, the Jewish historian Josephus too, writing of the Siege of Jerusalem in the year 70, recounted that the Jews caught outside the city walls "were first whipped, and then tormented with all sorts of tortures, before they died, and were then crucified before the wall of the city … the soldiers, out of the wrath and hatred they bore the Jews, nailed those they caught, one after one way, and another after another, to the crosses, by way of jest.
Dionysius of Halicarnassus, who lived at the time of the birth of Jesus, described how those condemned to crucifixion were led to the place of execution: "A Roman citizen of no obscure station, having ordered one of his slaves to be put to death, delivered him to his fellow-slaves to be led away, and in order that his punishment might be witnessed by all, directed them to drag him through the Forum and every other conspicuous part of the city as they whipped him, and that he should go ahead of the procession which the Romans were at that time conducting in honour of the god. The men ordered to lead the slave to his punishment, having stretched out both his arms and fastened them to a piece of wood which extended across his breast and shoulders as far as his wrists, followed him, tearing his naked body with whips.
Dionysius here uses the word ξύλον for the horizontal crossbeam (the "patibulum") used in Roman crucifixions; he describes how the hands of the condemned man were tied to it (χεῖρας ἀποτείναντες ἀμφοτέρας [...] προσδήσαντες) for him to be whipped while being led to the place of execution.
After quoting a poem by Maecenas that speaks of preferring life to death even when life is burdened with all the disadvantages of old age or even with acute torture Seneca states his disagreement with the sentiment, saying that for a crucified person hanging from the patibulum death would be better: "I should deem him most despicable had he wished to live to the point of crucifixion ... Is it worth so much to weigh down upon one's own wound, and hang stretched out from a patibulum? ... Is anyone found who, after being fastened to that accursed wood, already weakened, already deformed, swelling with ugly weals on shoulders and chest, with many reasons for dying even before getting to the cross, would wish to prolong a life-breath that is about to experience so many torments?" He refers to this patibulum or to the entire cross as "illud infelix lignum" (that accursed wood), using the Latin word "lignum" that corresponds to Greek "ξύλον".
Referring to what he saw as Old Testament intimations of Jesus and his cross, he likened the cross to the letter T (the Greek letter tau, which had the numeric value of 300), thus describing it as having a crossbeam.
He also wrote, with regard to : "The Spirit saith to the heart of Moses, that he should make a type of the cross and of Him that was to suffer, that unless, saith He, they shall set their hope on Him, war shall be waged against them for ever. Moses therefore pileth arms one upon another in the midst of the encounter, and standing on higher ground than any he stretched out his hands, and so Israel was again victorious.
Lucian of Samosata (121-180) describes the crucifixion of the mythical Prometheus by nailing him to a precipice on the Caucasus "with his hands outstretched (ἐκπετασθεὶς τὼ χεῖρε) from crag to crag.
The second-century Odes of Solomon, probably by a heterodox Christian, includes the following: "I extended my hands and hallowed my Lord, /For the expansion of my hands is His sign. /And my extension is the upright cross (σταυρός).
Justin Martyr (100–165) explicitly says the cross of Christ was of two-beam shape: "That lamb which was commanded to be wholly roasted was a symbol of the suffering of the cross which Christ would undergo. For the lamb, which is roasted, is roasted and dressed up in the form of the cross. For one spit is transfixed right through from the lower parts up to the head, and one across the back, to which are attached the legs of the lamb.
Like the Epistle of Barnabas, Justin saw the stretched-out hands of Moses in the battle against Amalek as foreshadowing the cross of Jesus: "If he gave up any part of this sign, which was an imitation of the cross (σταυρός), the people were beaten, as is recorded in the writings of Moses; but if he remained in this form, Amalek was proportionally defeated, and he who prevailed prevailed by the cross (σταυρός). For it was not because Moses so prayed that the people were stronger, but because, while one who bore the name of Jesus (Joshua) was in the forefront of the battle, he himself made the sign of the cross (σταυρός).
In his First Apology, 55 Justin refers to various objects as shaped like the cross of Christ: "The sea is not traversed except that trophy which is called a sail abide safe in the ship … And the human form differs from that of the irrational animals in nothing else than in its being erect and having the hands extended, and having on the face extending from the forehead what is called the nose, through which there is respiration for the living creature; and this shows no other form than that of the cross (σταυρός)."
The apocryphal Acts of Peter, of the second half of the second century, attaches symbolic significance to the upright and the crossbeam of the cross of Jesus: "What else is Christ, but the word, the sound of God? So that the word is the upright beam whereon I am crucified. And the sound is that which crosseth it, the nature of man. And the nail which holdeth the cross-tree unto the upright in the midst thereof is the conversion and repentance of man.
Irenaeus, who died around the end of the second century, speaks of the cross as having "five extremities, two in length, two in breadth, and one in the middle, on which [last] the person rests who is fixed by the nails.
Hippolytus of Rome, writing about the blessing Jacob obtained from his father Isaac said: "The skins which were put upon his arms are the sins of both peoples, which Christ, when His hands were stretched forth on the cross, fastened to it along with Himself.
In his Octavius, Marcus Minucius Felix, responding to the pagan jibe that Christians worship wooden crosses – an indication of how the cross symbol was already associated with Christians – denies the charge and then retorts that the cross shape (a crossbeam placed on an upright) is honoured even by pagans in the form of their standards and trophies and is in any case found in nature: "Crosses, moreover, we neither worship nor wish for. You, indeed, who consecrate gods of wood, adore wooden crosses perhaps as parts of your gods. For your very standards, as well as your banners; and flags of your camp, what else are they but crosses gilded and adorned? Your victorious trophies not only imitate the appearance of a simple cross, but also that of a man affixed to it. We assuredly see the sign of a cross, naturally, in the ship when it is carried along with swelling sails, when it glides forward with expanded oars; and when the military yoke is lifted up, it is the sign of a cross; and when a man adores God with a pure mind, with hands outstretched. Thus the sign of the cross either is sustained by a natural reason, or your own religion is formed with respect to it.
In language very similar to that of Minucius Felix, Tertullian, too, who distinguished between stipes (stake) and crux (cross), noted that it was the cross that people associated with Christianity. And he indicated that the shape of the cross is that of the letter T: "The Greek letter Tau and our own letter T is the very form of the cross, which (God) predicted would be the sign on our foreheads", and compared it to the shape of a bird with outstretched wings.
The anti-Christian arguments thus cited in the Octavius of Minucius Felix, chapters IX and XXIX, and Tertullian's Apology, 16 show that the cross symbol was already associated with Christians in the second century. Tertullian could designate the body of Christian believers as crucis religiosi, i.e. "devotees of the Cross". In his book De Corona, written in 204, Tertullian tells how it was already a tradition for Christians to trace repeatedly on their foreheads the sign of the cross.
So closely associated with Christ was the cross that Clement of Alexandria, who died between 211 and 216, could without fear of ambiguity use the phrase τὸ κυριακὸν σημεῖον (the Lord's sign) to mean the cross, when he repeated the idea, current as early as the Epistle of Barnabas, that the number 318 (in Greek numerals, ΤΙΗ) in was a foreshadowing (a "type") of the cross (T, an upright with crossbar, standing for 300) and of Jesus (ΙΗ, the first two letter of his name ΙΗΣΟΥΣ, standing for 18).
For other second-century instances of the use of the cross, in its familiar form, as a Christian symbol, see the references in the Jewish Encyclopedia article on the cross:
In his article "Anthropological Observations on the Skeletal Remains from Giv’at ha-Mivtar" published in the Israel Exploration Journal in 1970, N. Haas of the Department of Anatomy at Hebrew University, writes of the remains of a man crucified around A.D. 70 as showing that the two-beam cross was in use in Palestine in the first century: "The whole of our interpretation concerning the position of the body on the cross may be described briefly as follows: The feet were joined almost parallel, both transfixed by the same nail at the heels, with the legs adjacent; the knees were doubled, the right one overlapping the left; the trunk was contorted; the upper limbs were stretched out, each stabbed by a nail in the forearm.