Although the Greek word “cross” (stauros) originally meant just a stake, and the Romans kept large stakes permanently planted at places of execution, the available textual evidence from the Roman period points strongly to the use on the stake of a patibulum or crossbeam in crucifixions to spread and suspend the victim’s arms. Although the basic shape of the “cross” was probably therefore a T-shape, the exact shape of the cross would depend on the mechanics of how the crossbeam was anchored to the upright.
The two main possibilities are:
(a) a hole at the top of the stake, through which the crossbeam was inserted – in Latin a crux immissa (inserted)
(b) a hole in the crossbeam, to enable it to sit securely on top of the stake – in Latin a crux commissa (put on)
In practical terms both of these possibilities produce a similar result: something between a T-shape and the traditional cross, because the top of the stake would need to protrude above the basic T-shape to some degree, but a few inches would have been enough above the basic T-shape to keep the crossbeam in place.