Why did David make all the Moabites line up in a row?

ANSWER

to kill every two and then let the third go free

2nd Samuel 8 :

2 David also defeated the Moabites. He made them lie down on the ground and measured them off with a length of cord. Every two lengths of them were put to death, and the third length was allowed to live. So the Moabites became subject to David and brought him tribute.
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