On the one side … on the other side: that is, on the two sides of the Valley of Elah mentioned in the previous verse.

A valley: the Hebrew word is not the same as that translated “valley” in verse 2. The Hebrew word here refers to a gorge or “ravine” (so New Jewish Publication Society’s Tanakh and Klein) cut out by a stream of water running down the valley of Elah. Both Revised Standard Version and Good News Translation use the indefinite article, “a [valley],” though Hebrew says “the valley [or, ravine].” Use of the indefinite article “a [valley]” in English gives the wrong impression that the valley separating the two armies is different from the valley mentioned in verse 2. New Jerusalem Bible and La Bible du Semeur more clearly say “the valley.”

Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on the First and Second Books of Samuel, Volume 1. (UBS Helps for Translators). New York: UBS, 2001. For this and other handbooks for translators see here .