Might it not, then, be that "stauros" referred mainly to the crossbeam,
and "ksulon" to the upright? That might explain why ksulon, "tree", was
used occasionally (to which Jesus was affixed, that is, to the upright,
cf. Acts in the mouth of Peter et alia) side by side with "stauros" in
the sense of the mechanism of his crucifixion (nailed to the crossbeam
only?). I'm just completely guessing, though.
But I'm still kind of lost about how a message could be put above Jesus
if he was on a Tau cross or a Chi cross (what would it be attached to? A
stake maybe). Am I missing something obvious here? And yet it does seem
to imply that the cross was quite short - not far above head level, despite
the usual image of it towering above the landscape.