Instrument of Jesus's crucifixion
The instrument of Jesus's crucifixion (known in Latin as crux, in Greek as stauros) is generally taken to have been composed of an upright wooden beam to which was added a transom, thus forming a "cruciform" or T-shaped structure. Most Christian denominations present the Christian cross in this form, and the tradition of the T-shape can be traced to early Christianity and the Church Fathers.
Source: Wikipedia — Instrument of Jesus's crucifixion (CC BY-SA 4.0)