Every piece of timber which is fixed in the ground in an erect position is a part of a cross, and indeed the greater portion of its mass. But an entire cross is attributed to us, with its transverse beam, of course, and its projecting seat.
…Since the head rises upwards, and the back takes a straight direction, and the shoulders project laterally, if you simply place a man with his arms and hands outstretched, you will make the general outline of a cross.
Tertullian
Ad Nationes, Book I
Chapter XII