The symbol originates from an evolution of the initial letter Theta, , of the Ancient Greek name () of the planet. It was adopted in Late Antiquity as an astrological symbol for the planet, and hence as an old and obsolete alchemical symbol for iron. Medieval conjecture by Scaliger was that its current form represents spear and shield, derived from a medieval form where the spear was drawn across the shield, which in turn was based on a convention used in antiquity ("Bianchini's planisphere") which represented Mars by a spear. This conjecture was also discredited by Salmasius, whose analysis is supported by modern scholarship.