The Last Letter
The Hebrew alphabet ends with taw. Twenty-two letters, and taw is the twenty-second — the closing signature of the alphabet. In the Sinaitic script (the earliest stage of the alphabet, c. 1500 BC, attested in inscriptions at Serabit el-Khadim and Wadi el-Hol), taw was drawn as a simple cross or X. The shape is plain and load-bearing: two strokes intersecting, the most basic possible "mark."
The form is conserved across the early Semitic family. Phoenician inscriptions of the 11th–9th centuries BC keep the X. Old Aramaic and the early Hebrew inscriptions (the Gezer Calendar, the Siloam tunnel inscription) show a slightly tilted X, the bottom-right stroke beginning to curl. Only after the Babylonian exile, when the square Aramaic script displaces paleo-Hebrew for sacred writing, does taw take its modern form: ת — closed on the right, open on the left, no longer recognizable as a cross.
That matters for one specific reason. When the prophet Ezekiel writes c. 593–571 BC and the man clothed in linen is told to "set a taw upon the foreheads of the men that sigh and that cry" (Ezek 9:4), the script the prophet would have used was paleo-Hebrew. The mark on the foreheads of the righteous in Jerusalem — by the most direct reading of the text and the script of its writing — was a cross.