Last week, there was a show on Torture & Death machines I watched a part of. The cross was included as one of those machines. The show was on Discovery channel.
The information presented that the Roman cross was actually 'T' shaped (Tau) and did not actually cross over as is common in the Latin cross, the cross shape we are most familiar.
The stake in the Roman cross was 10 - 12' long with approx. 1/3 buried permanently in the ground, so as to be used for multiple crucifixions. The top of the stake had a tenon (smaller dimensioned protrusion cut from the larger stake) which would fit into a socket (mortise) cut into the centre of a 6’ cross beam (patibulum). In this manner, the cross beam was actually balancing on top of the stake with the mortise/tenon joint securing the two pieces together with no other fastening method needed.
The condemned would be nailed to the cross beam while the beam was on the ground. To hang the beam was simply a matter of hoisting the beam (with prisoner) to the top of the stake and inserting the tenon into the mortise.
Nailing the condemned is usually most effective when done through the wrist, between the larger bones as this area would be able to support the persons weight while less likely to rip as would be likely if nailed through the hands. Nailing the hands would result in more pain but ropes would have had to be used to lash the persons arms to the cross beam for added support.
It was discussed that utilizing a cross beam was far more torturous than fastening the hands straight overhead. As an example, in gymnastics, a gymnast hanging by overhead bar or rings, can comfortably stay hanging longer, than a gymnast doing the ‘Iron cross’ with arms straight out.
They also showed that the feet were normally nailed to the sides of the stake; they had depicted the nails going through the ankles. It said that the crucified would have difficulty breathing so they would often push up using their legs to help exhale. It was stated that this was the cruellest part of the process. By nailing the feet in the manner just described, when the person would push up, injured nerves in both the ankle/feet and wrist/hands would be reactivated, resulting in intense pain shooting through the entire body making breathing part of the torture process.
Described with the physical description of this machine, its location and oriention were planned to give the condemned the most weather exposure (sun, wind etc) to help wear them down.
Gravity was as the main force that caused torture & ultimately death. Since the crucified slumps lower and lower as time passes, a sign fastened to the crossbeam would be located above the condemned persons head.