John Gill Bible Commentary

And the Philistines stood on a mountain on the one side, and Israel stood on a mountain on the other side, Before the Israelites are said to encamp in or by the valley; but here they are said to take the higher ground, and face the Philistines, who were on a mountain or hill on the other side over against them, which Kimchi reconciles thus; the whole or the grand army lay encamped in the valley, and, they that were set in array, or the first ranks, the first battalion, ascended the mountain to meet the Philistines. Vatablus takes it to be the same mountain, that on one part of it the Philistines formed their first battalion, and the rest of the army was in the valley; and on the other part of the mountain the Israelites pitched their camp: and there was a valley between them; the same as in the preceding verse.