Most standard resources state that the earliest _stauros_ was a vertical
stake called _stauros_ or _skolops_ (the latter is the term Celsus applied
to the "cross" of Christ). Among other forms are the "Tau" cross (T),
also called crux commissa, the "latin" cross or crux immissa, the "St.
Andrew's" cross, or crux decussata (x), and there were other forms as
well. As Seneca wrote, "I see crosses there; not just one kind, but made
in many different ways."
One source suggests that the most likely (as that intended in the NT) is
the crux immissa or "latin" cross, because of the inscription nailed above
the head of Christ. That is not entirely logical, however, because that
would easily be possible on both the simple post and the "T"-shaped "Tau"
cross. The "Tau" cross seems to have been very prominent in the early
years of Christianity, and as late as the time of Luther one finds many
paintings showing either Christ or those crucified beside him placed upon
a "Tau" cross.