The answers here have been reconstructed from the original contributors, and edited for spelling and content. This answer may be added to, but must remain as is otherwise.
The portions of the Bible that contain the account of Jesus death were originally written in Greek. To understand what the instrument that was used for his execution was, we have to realize what the original Greek words that were used, mean. The two words that describe the executional instrument were: "stauros" and "xylon."
The word "stauros" means an upright pale or stake. The Companion Bible points out: "[Stauros] never means two pieces of timber placed across one another at any angle. There is nothing in the Greek of the [New Testament] even to imply two pieces of timber. "
The Imperial Bible-Dictionary acknowledges this, saying: "The Greek word for cross, [stauros], properly signified a stake, an upright pole, or piece of paling, on which anything might be hung, or which might be used in impaling [fencing in] a piece of ground...Even amongst the Romans the crux (from which our cross is derived) appears to have been originally an upright pole."--Edited by P. Fairbairn (London, 1874), Vol. I, p. 376. [This paragraph was taken directly from the Watchtower publication, Reasoning from the Scriptures. In quoting from the Imperial Bible-Dictionary, the publishers omitted the following by the use of ellipses: "But a modification was introduced as the dominion and usages of Rome extended themselves through Greek-speaking countries." In addition, "pole" was not the end of the sentence quoted. It was followed by a comma, and continued, "and this always remained the more prominent part. But from the time that it began to be recognized as an instrument of punishment, a transverse piece of wood was commonly added; not, however, always even then." After identifying three forms of crosses, the entry continues, "There can be no doubt, however, that the latter sort (extending their arms on a patibulum) was the more common, and that about the period of the gospel age crucifixion was usually accomplished by suspending the criminal on a cross piece of wood." After further identifying three different cross shapes, the entry continues, "But the commonest form, it is understood, was that in which the upright piece of wood was crossed by another near the top, but not precisely at it...It was on a cross of this form, according to the general voice of tradition, that our Lord suffered; but there is nothing in the narratives of the evangelists which determines this to have been the form employed, rather than either of the other two. It is, however, the one most commonly met with in the paintings and sculptures that have survived from the earlier ages."]
The other word, "xylon" means simply, 'a timber or a stick, club, or tree.' It denotes the center stalk of the tree with all it's branches lopped off.
Explaining why a simple stake was often used for executions, the book Das Kreuz und die Kreuzigung (The Cross and the Crucifixion), by Hermann Fulda, states: Trees were not everywhere available at the places chosen for public execution. So a simple beam was sunk into the ground. On this the outlaws, with hands raised upward and often also with their feet, were bound or nailed. [Pastor Fulda was the first person to suggest that Jesus was executed on a stake in this book written in 1878. However, Fulda based his assumption on the etymology of the Greek word "stauros" and Greek custom. We should rather look to Roman methods of execution since how the Greeks did it is irrelevant.]
Another point to consider is that a piece of timber with a crossbar would not be a feasible means of execution. The Roman method of impalement worked by suffocation. With the victim's arms suspended above them, the airways become restricted. That is why we are told to never suspend an infant by the arms. It becomes harder to breath. As the victim's legs become weaker and weaker, they can no longer push themselves up to get each gasp of air. They die by suffocation. That is why the Romans would break the legs of the victims to speed up the death, if the process were to delay. With a cross piece attached, and the arms outstretched, the victim could then suspend themselves on their outstretched arms and keep breathing indefinitely. [The April 1989 issue of Bible Review contains an article entitled "Two Questions About Crucifixion." The conclusion of Frederick T. Zugibe, adjunct associate professor of pathology at Columbia University College of Physicians and Surgeons, was that a man nailed to a stake with arms stretched straight over his head (as the Watchtower depicts) would suffocate in minutes. This confirms earlier research done by A. A. LeBec in 1925, Austrian radiologist, Hermann Moedder in the 1940s and Dr. Pierre Barbet in 1953, who all concluded that a person hung by his arms overhead would suffocate in a manner of minutes (Moedder proposed precisely six minutes), due to the inability of the lungs to expand and contract in such a position. Yet the Bible account depicts Jesus on the cross for seven hours before dying.]
The book The Non-Christian Cross, by J. D. Parsons (London, 1896), says: "There is not a single sentence in any of the numerous writings forming the New Testament, which, in the original Greek, bears even indirect evidence to the effect that the stauros used in the case of Jesus was other than an ordinary stauros; much less to the effect that it consisted, not of one piece of timber, but of two pieces nailed together in the form of a cross. It is a little misleading upon the part of our teachers to translate the word stauros as cross when rendering the Greek documents of the Church into our native tongue, and to support that action by putting cross in our lexicons as the meaning of stauros without carefully explaining that that was at any rate not the primary meaning of the word in the days of the Apostles, did not become its primary signification till long afterwards, and became so then, if at all, only because, despite the absence of corroborative evidence, it was for some reason or other assumed that the particular stauros upon which Jesus was executed had that particular shape."(Pp. 23,24) ;(see also The Companion Bible ,London, 1885, Appendix No. 162.) [John 20:25: "Unless I shall see in His hands the imprint of the nails, and put my finger into the place of the nails..." The Bible affirms twice that at least two nails were used to nail Jesus' hands to the cross. Mat. 27:37: "And they put up above His head the charge against Him..." Matthew describes the inscription as being above Jesus' head, not His hands. Mat. 27:32: "And as they were coming out, they found a man of Cyrene named Simon, whom they pressed into service to bear His cross." "The upright post to which alone the name [stauros] properly belongs, was usually a piece of some strong, cheap wood, pine or oak, of such length that when firmly planted in the ground the tip was from 7 1/2 to 9 ft. high. erected on some spot out-side the city, convenient for the execution, and remained there as a permanent fixture, only the cross-bar or 'patibulum' being carried to the spot, usually by the person who was to suffer death. A Dictionary of the Bible, ed. J. Hastings (T & T Clark, 1951), "Cross," p. 528.]
Interestingly, the word "crux" (meaning cross) does not appear in the oldest manuscripts of the Bible. That word does not appear in Bible texts, until several centuries after the death of Jesus and the apostles. [Perhaps this is because "crux" is a Latin word, and the New Testament was written in Koine Greek.]
The apostle Paul says: "Christ by purchase released us from the curse of the Law by becoming a curse instead of us, because it is written: 'Accursed is every man hanged upon a stake [a tree, King James Version]." (Galatians 3:13) Here Paul quotes Deuteronomy 21:22, 23, which clearly refers to a stake, not a cross. Since such a means of execution made the person a curse, it would not be proper for Christians to decorate their homes with images of Christ impaled. [Here Paul is not giving a description of the instrument of Jesus' death, but is drawing a parallel with the passage from Deuteronomy.]
There is no evidence that for the first 300 years after Christ's death, those claiming to be Christians used the cross in worship. In the fourth century, however, pagan Emperor Constantine claimed to convert to Christianity and promoted the cross as its symbol. Whatever Constantine's motives, the cross had nothing to do with Jesus Christ. The cross is, in fact, pagan in origin. The New Catholic Encyclopedia admits: "The cross is found in both pre-Christian and non-Christian cultures." Various other authorities have linked the cross with nature worship and pagan sex rites. [In 1873 a famous French scholar, Charles Clermant-Ganneau, reported the discovery of a burial chamber or cave on the Mount of Olives. Inside were some 30 ossuaries (rectangular chests made of stone) in which skeletal remains were preserved after their bodies had disintegrated...One (ossuary) had the name "Judah" associated with a cross with arms of equal length. Further, the name "Jesus" occurred three times, twice in association with a cross...It would be unlikely that Christian Jews would have been buried in that area after 135 A.D. since the Romans forbade Jews to enter Aelia Capitolina...after the second Jewish revolt. (from Ancient Times, Vol. 3, No. 1, July 1958, p. 3.) In 1939 excavations at Herculaneum, the sister city of Pompeii (destroyed in 78 A.D. by volcano) produced a house where a wooden cross had been nailed to the wall of a room. According to Buried History, (Vol. 10, No. 1, March 1974 p. 15): "Below this (cross) was a cupboard with a step in front. This has considered to be in the shape of an ara or shrine, but could well have been used as a place of prayer. . . . If this interpretation is correct, and the excavators are strongly in favor of the Christian significance of symbol and furnishings, then here we have the example of an early house church." In 1945 a family tomb was discovered in Jerusalem by Prof. E.L. Sukenik of the Museum of Jewish Antiquities of the Hebrew University. Prof. Sukenik is the world's leading authority on Jewish ossuaries. Note his findings: "Two of the ossuaries bear the name 'Jesus' in Greek...The second of these also has four large crosses drawn...(Prof. Sukenik) concluded that the full inscriptions and the crosses were related, being expressions of grief at the crucifixion of Jesus, being written about that time...Professor Sukenik points out...(that) the cross may represent a "pictorial expression of the crucifixion, tantamount to exclaiming `He was crucified!'" As the tomb is dated by pottery, lamps and the character of the letters used in the inscriptions--from the first century B.C. to not later than the middle of the first century A.D. this means that the inscriptions fall within two decades of the Crucifixion at the latest. (Ancient Times, Vol. 3, No. 1, July 1958, p. 35. See also Vol. 5, No. 3, March 1961, p. 13.)
Why, then, was this pagan symbol promoted? Apparently, it was done to make it easier for pagans to accept the Church. Nevertheless, devotion to any pagan symbol is clearly condemned by the Bible. (2 Corinthians 6:14-18) The Scriptures also forbid all forms of idolatry. (Exodus 20:4,5; 1 Corinthians 10:14) [Scientific and medical evidence, passages of Scripture, history and archeology all identify that Jesus died on a cross. Linguistics supports that stauros meant cross prior to the first century. The cross was identified with early Christianity and is not pagan. It is simply false to say it was introduced 300 years after Christ by Constantine. That some Christians have misused the cross as a symbol for worship is not justification for rejecting accurate history. If it was appropriate for Paul to boast in the cross of our Lord (Gal. 6:14), it is acceptable for Christians today as well.
Two methods were followed in the infliction of the punishment of crucifixion. In both of these the criminal was first stripped naked, and bound to an upright stake, where he was scourged. After this, the victim was dressed again, and, if able, was made to drag the cross or a part of it (usually weighing 150 lb or more) to the place of execution. At this point he was again stripped naked, and was either fastened to it or impaled upon it, and left to die. In this method, the crux simplex of Justus Lipsius, a single stake was used. [Justus Lipsius had a number of wood carvings depicting various methods of crucifixion in his book. He noted that the earliest form was on a stake, but pictured Jesus on the traditional cross.
Despite the fact that the ancient Jewish historian Josephus, as well as other sources, refer to the crucifixion of thousands of people by the Romans, there is only a single archaeological discovery of a crucified body dating back to the Roman Empire around the time of Jesus which was discovered in Jerusalem. It is not surprising that there is only one such discovery, because a crucified body was usually left to decay on the cross and therefore would not be preserved.
Firstly let's consider the symbolism of the Cross in the early Church. The cross symbol was already associated with Christians in the second century, as is indicated in the (anti-Christian, so not biased towards Christianity by any means!) arguments cited in the Octavius of Minucius Felix, chapters IX and XXIX, written at the end of that century or, possibly the very beginning of the next. Minucius Felix speaks of the cross of Jesus in its familiar form, likening it to objects with a crossbeam or to a man with arms outstretched in prayer.
Also, Clement of Alexandria, (q.v.) who died between 211 and 216, frequently used the phrase τὸ κυριακὸν σημεῖον (the Lord's sign) to mean the cross, and there are several citations in his work to corroborate this. In his writings he describes the Cross as a symbol of the Christian - a Cross that was made by a vertical line crossed by a horizontal one.
His contemporary, the great theologian Tertullian described the body of Christian believers as crucis religiosi, i.e. "devotees of the Cross". In his book De Corona, Chapter 3, written in 204, Tertullian tells how it was already a tradition for Christians to trace repeatedly on their foreheads the sign of the cross as a mark of their devotion to Christ.
Another early Church father, who lived from 130-202, the great Irenaeus, wrote Adversus Haereses where in section II, xxiv, 4 said these words: ."The very form of the Cross, too, has five extremities, two in length, two in breadth, and one in the middle, on which the victim rested who is fixed by the nails". Irenaeus was a pupil if Polycarp, who was, in turn, a pupil of St John the Apostle, the only disciple cited in scripture that was actually present at the crucifixion. Hence, Irenaeus would hardly have made an error in the cross shape and therefore, the cross, as described by him, must be given serious consideration. <?xml:namespace prefix = o ns = "urn:schemas-Microsoft-com:office:office" Secondly, let's look at the historical and archaeological evidence of the Roman Empire.
The term 'crucifixion' comes from the Latin crucifixio ("fixed to a cross", from the prefix cruci-, "cross", + verb ficere, "fix or do") and not 'fixed to a stake'. Very occasionally, the gibbet was only one vertical stake, called in Latin crux simplex or palus, or in Greek μόνος σταυρός (monos stauros, i.e. isolated stake) but never simply σταυρός stauros, which invariably was reserved for a cross shaped structure, and not, as Jehovah's Witnesses continually assert in error- a stake. The monos stauros was the simplest available construction for torturing and killing criminals. More often, however, there was a cross-piece attached either at the top to give the shape of a T (crux commissa) or just below the top, as in the form most familiar in Christian symbolism (crux immissa).Other forms were in the shape of the letters X and Y.
Shapes of cross tended to be local, so that, say, a stake would be favored in one part of the Empire, whilst a cross would be used in another.The earliest writings that speak specifically of the shape of the cross on which Jesus died describe it as shaped like the letter T (the Greek letter tau), or, more likely, composed of an upright and a transverse beam like a lower case Tau (t), together with a small ledge in the upright.
The apocryphal Epistle of Barnabas, Chapter 9 mentions this, and, although not a canonical gospel, still undoubtedly belongs to the end of the first or beginning of the second century and contains a great deal of contemporary information that has to be taken seriously.
Furthermore, archeological discoveries made in the 1960s of a Christian cross being found in Herculaneum around 70AD backs up the use of a Cross for crucifixion rather than a stake - at least in Italy. The Cross, as well as the fish, the star, the Chi-Rho sign, the anchor and the plough, are to be found on second century ossuaries of the Judaeo-Christian community in Judaea which put any doubt as to the nature of the Cross shape beyond all reach. Michael Green, an expert on the early church, confirms this in "Evangelism in the Early Church" pp. 214-215. In June of 1968, some 1st century tombs were accidentally unearthed by bulldozers working north of Jerusalem.
Archaeologist, Vasilius Tzaferis excavated the tombs and unearthed the skeletal remains of a young man who had been crucified, the position and type of wound strongly confirming crucifixion on a Cross and not a stake. Mr Tzaferis, who is not a Christian, wrote an article on his findings in the Biblical Archaeological Review. As the crucified man was crucified on a cross-shaped cross, this suggested strongly that the Cross was the preferred local method for crucifixion in those parts - i.e. Jerusalem, suggesting strongly that Christ was crucified on such a cross because of the nature of Roman custom in its locality-specific preferences as to the type of execution structure.
Also, in local contemporary graffiti, there is an anti-Christian picture that shows a Christian believer worshipping an ass headed god on a cross. The figure of Christ is crucified with his arms outstretched - graffiti drawn by a contemporary figure. Finally, within the catacombs in Rome, the Cross symbol is seen frequently as a symbol for Christianity, many of which date from the earliest Christian church there. I saw these symbols scratched onto walls, on tombs and even a stone altar myself whilst on a visit there a few years ago. All of the Cross symbols were of the standard Christian Cross as an upright and cross beam. Nowhere was there any hint of a 'stake' being used symbolically for an event which, along with the resurrection and ascension, was the pivotal belief of the early Church and the one to which Paul referred time and time again.
Finally, but certainly not least, we should turn to scripture. In representations of Christ on a torture stake, Jehovah's Witnesses always show 'one nail' through both of Christ's hands. However, in contrast to this, the Bible clearly states that Christ's crucifixion bore the marks of two nails and not one. This is seen in John 20:25 which records Thomas as saying: "...Unless I see in His hands the imprint of the nails, and put my finger into the place of the nails, and put my hand into His side, I will not believe." Note that the plurality of the nails refers to the hands only, and not the hands and feet. This is evident both in the English account and the original Greek. In addition to this text, further biblical evidence that Christ was crucified on a cross, rather than a torture stake, can be gleaned from Matthew 27:37, which describes the charge placed "above" Christ's "head" which read: "This is Jesus The King of the Jews". Pilate, in doing so and in insisting this was the charge, and not "He said he was the King of the Jews", would have meant that the charge was meant to be seen by all as a statement of Pilate's defiance, although a weak defiance, of the crowd. Again, the Greek is specific - the charge was placed above Christ's head. If Christ had been crucified on a torture stake, the charge would have been unreadable, as His hands would have obstructed the words. If it was placed higher up the pole, it may have been more appropriate for Matthew, normally a stickler for detail, to have written that the charge was placed above His hands. But he didn't. The traditional historic view of Christ being crucified on a cross, would both make the charge easier for all to have seen, and, as the text says, been placed above His head. In addition to this, all four gospels record Jesus carrying his Cross or Cross-beam. In, e.g. John 19:17, the word used is stauros "και βασταζων εαυτω τον σταυρον εξηλθεν εις τον λεγομενον κρανιου τοπον ο λεγεται εβραιστι γολγοθα" But this does not mean that the word 'stauros means a stake in this context, or it would have been μόνος σταυρός (monos stauros) instead, as explained above. Certainly the earliest Latin Bible translators thought of the Cross as just that - so that the word 'Crucis' was used rather than anything else in the translation - especially in the Latin Vulgate from which many translations, including the King James' version (which JWs use as well as the NWT) were taken. The reasons for the misconception about the word stauros are too many and complex to go into, but there is an excellent unbiassed article that examines the original Greek etymology of the word and the Jehovah's Witness claims. Finally, Paul, in many of his letters, refers to Jesus being crucified on a Cross, and never on a stake. It seems that Jehovah's Witnesses over recent decades, have shown an almost superstitious dread and fear of the cross shape. So convinced are they, of what the Watchtower has told them regarding this image, that many instantly associate it with paganism. This is very silly, especially as the JWs believe in the infallibility of scripture and their belief in exclusivity in its correct interpretation. Yet their founder, Charles Taize Russell, (who, incidentally based his theology on heretical Arian ideas, and who, in court could not even recite the Greek alphabet despite his fraudulent claims to be a Greek scholar) and his followers esteemed the cross as a symbol of Christ's redemption of mankind from sin, and even published the cross-and-crown image on Watchtower covers and wearing it as a clothing pin! Carey W. Barber, later a member of the Governing Body of the Watchtower Society, described the pin: "It was a badge really, with a wreath of laurel leaves as the border and within the wreath was a crown with a cross running through it on an angle. It looked quite attractive and was our idea of what it meant to take up our cross and follow Christ Jesus in order to be able to wear the crown of victory in due time" (1975 Yearbook, p. 148)". There seems to be a great deal of confusion about what JWs actually believe. Russell believed, and wore, a Cross. Then in 1931 Rutherford - another high ranking JW removed it from Watchtower publications and declared that it was a pagan symbol. Being uncomfortable that JWs still regarded Jesus as being crucified on a Cross, he then declared (with absolutely no evidence whatsoever) that Jesus was crucified on a tree (Rutherford: Riches p 27). Now they have the idea that Jesus was nailed to a stake, again with little real evidence. It seems therefore, that the evidence, archaeological, historical, geographical and scriptural all points to a Cross on which Christ was crucified. Typically, for ever being bogged down in needless detail and missing the whole picture, JWs still argue just what sort of Cross it was, and continue to change their minds as they have done over so many doctrines, including the Trinity, and predictions for the end of the world, all of which have come to naught because they simply will not adhere to Christ's teaching on it in Matthew's (and the other writers') gospel account(s). To Christians, they see the sad JW movement as the foolish man not only building their house on the sand, but on shifting sand at that. To most Christians the important part as not what shape the structure was upon which Christ was crucified, but rather what happened there. Certainly, theologians and scholars have no doubt pondered on the shape of the implement of crucifixion, but it has never been an issue of primary concern. However, it should be remembered that it was not the Christian Church, from its very beginnings until today, who began to attack the shape of the cross, and rejected it as a pagan symbol, but rather the Watchtower Society alone, against all evidence, historical, archaeological and scriptural; a society that claims almost Gnostic-like exclusivity of scriptural interpretation, but a society that has changed its mind, not only on the peripherals of religion but even on the basics over the years, and, as such lacks any credibility. While it is wrong to judge a JW as a person, as Christians are called to love one another whatever the circumstances, it is not wrong to judge the Society's beliefs which still follow Arian heresy that was discredited way back in the early Church. As far as the Cross is concerned, Christians are simply lauding a symbol that dated from Christ himself which reminds them of an event which is so, so important to them.