Charles Ellicott Commentary 1 Samuel 17:19

"Now Saul, and they, and all the men of Israel, were in the vale of Elah, fighting with the Philistines." — 1 Samuel 17:19 (ASV)

In the valley of Elan, fighting with the Philistines. —The words of this verse, which read in the English Version as an explanatory parenthesis, are really part of Jesse’s direction to his shepherd son, telling him where he would find his brothers. “They are in the valley of the Terebinth (Elah), fighting with the Philistines.”