The Bible
While some translations (e.g. New World Translation) use expressions like "torture stake", most English translations of the Bible refer to Jesus's execution device as a "cross". The original Greek word is "stauros" (σταυρός), which simply means an upright wooden stake, and has no connotation of having a crossbar.
Jesus, or anyone else, couldn't possibly have carried anything like the large and often elaborate cross we see depicted in churches, books, and films. Nor was he forced to carry only the crossbar, as some people now rationalize the event. The same Greek word is used for what he carried as is used for the execution device.
Even if the Romans had thought of Jesus as deserving special treatment because of who he was (which they didn't), they obviously didn't bother. Jesus was crucified alongside two other criminals, with nothing mentioned indicating that his stake was any different from the other two.
The soldiers were asked to break the legs of the three criminals so that they would die much sooner allowing them to be buried before the Sabbath began at sunset. The leg breaking would speed up the death because the entire body weight would then be on the single nail between the crossed wrists. The pain would be intense and continuous, and any false hope provided by leg support would be gone.
Why There is nothing recorded in the first few centuries to indicate a cross-shaped execution device, so where did this iconic shape originate?
In 312CE, when the Roman emperor Constantine won the battle of Milvian Bridge, he saw a cross of light and the message "In this sign you shall conquer".
This might have been the meteorological phenomenon known as a sundog: vertical and horizontal beams of light centered around the sun with four similar but smaller crosses occurring on a ring of light surrounding the sun.
Even today, many Roman symbols of the cross look much like this phenomenon.
From ancient times, cross symbols had been used by many cultures to represent the Sun. Constantine was a sun worshipper, so invoking the power of the Sun god in battle would have been a natural thing for him to do. The Emperor ordered his soldiers to put the cross symbol on their shields in all future battles.
Over the next decade, because his mother had become interested in Christianity, Constantine relaxed the official persecution of Christians, and began to see the potential usefulness of that cult. He gradually incorporated Christian terminology into the Roman religion and convened councils to set doctrine, including making Sunday, the day of the Sun, the official day of rest for Christians. Later he outlawed the Biblical holidays and declared that the Roman celebrations that we now know as Easter and Christmas must be celebrated instead.
It was a simple matter to say that the device of Jesus's execution was a cross, and the Latin cross, the sign of the Sun, became the official emblem of Roman Christianity. This transition is supposedly supported by Malachi 4:2, which, when considered as a messianic prophecy, refers to Christ as the "sun of righteousness".
It wasn't until 25 years after his original Sun vision, and years of redefining Roman religion, that Constantine himself finally converted to what was by then called "Christianity".