Descriptions in antiquity of the execution cross
Descriptions in antiquity of the execution cross, whether by Christians or non-Christians, present the instrument ordinarily used in putting people to death by crucifixion as composed of two wooden pieces. Whether the two pieces of timber of the normal execution cross were permanently conjoined or were merely put together for the purpose of the execution is not stated.
Justus Lipsius on this page distinguishes two types of crux simplex
Justus Lipsius: Crux simplex ad infixionem
Justus Lipsius: Crux decussata, a form of cross for which there is no evidence of its use by ancient Romans: the earliest account of its supposed use for Andrew the Apostle is of the twelfth century.
Noah in prayer posture, a fresco in the catacombs of Rome
A cross is a geometrical figure consisting of two intersecting lines or bars, usually perpendicular to each other. The lines usually run vertically and horizontally. A cross of oblique lines, in the shape of the Latin letter X, is termed a saltire in heraldic terminology.
Bronze Age "wheel pendants" in the shape of the "sun cross" (Urnfield culture, 2nd millennium BC).
Early use of a globus cruciger on a solidus minted by Leontios (r. 695–698); on the obverse, a stepped cross in the shape of an Iota Eta monogram.