Albert Barnes Commentary 1 Samuel 21:13

"And he changed his behavior before them, and feigned himself mad in their hands, and scrabbled on the doors of the gate, and let his spittle fall down upon his beard." — 1 Samuel 21:13 (ASV)

Scrabbled - Literally, made marks (margin), namely, the mark of the “tau” (t), which in the ancient Hebrew and Phoenician was in the shape of a cross .

On the doors of the gate - The gate of Achish’s palace-yard or court, where the attendants waited. The house itself stood in this court (Compare Esther 2:19, 21).