สัญลักษณ์กางเขนThe crosscrux

Two intersecting lines are the cheapest mark a hand can make, so the shape has no single origin. Cross-shaped incisions appear from deep prehistory, as petroglyphs in European cult caves from the beginning of the Upper Palaeolithic and right through to the Iron Age. The cross mark also turns up as a number independently — Roman X for ten, Chinese rod numeral 十 for ten, the Brahmi numeral that becomes 4 — and in the Phoenician alphabet it is taw, whose name means mark.

The Christian cross is a later, dated adoption of an existing shape. It came into use as an emblem of Christianity from the second century CE, in the Latin letter T form, after Ichthys. Clement of Alexandria in the early third century calls it the Lord's sign. Tertullian, writing De Corona in 204, says it was already a tradition for Christians to trace the sign repeatedly on their foreheads — a cross made of a gesture before it is made of metal. Crosses with intersecting beams appear in Christian art towards the end of Late Antiquity; the patriarchal cross first appears in the tenth century; heraldry multiplies the forms from the Crusades onward.

What gets hung on a neck varies by church and by craft. The Coptic cross is two bold equal bars, each ending in three points for the Trinity, twelve points in all for the Apostles, often with a circle at the centre for God's love; the trefoil form developed locally from the short Greek cross. Many Copts have a cross tattooed on the inside of the right wrist as a sign of faith. In Ireland the cross is woven: the National Museum of Ireland sorts Brigid's crosses into seven categories by build rather than by outline, including a bent-armed type and a wheel type.

The Tuareg Agadez cross is the case that resists the word. Only a few tanaghilt actually resemble a cross; most are pendants with a varied silhouette, cast by blacksmiths in silver by lost-wax and never hammered. Asked what it means, users generally give vague answers; some call it a grigri or a reserve of wealth. The ethnologists Germaine Dieterlen and Ziedonis Ligers recorded a father giving the jewel to his son at the age of virility, marriage and nomadism with the words: my son, I give you the four directions of the world, because we do not know where you will go to die.

Inference — filing all of these under one sign is a convenience of shape, not a claim of common descent, and the Agadez piece is here mainly to show where the convenience breaks.