Then the boundary goes up by the Valley of the Son of Hinnom at the southern shoulder of the Jebusite (that is, Jerusalem). And the boundary goes up to the top of the mountain that lies over against the Valley of Hinnom, on the west, at the northern end of the Valley of Rephaim.
Judah and Joseph were the two sons of Jacob on whom Reuben's forfeited birth-right devolved. Judah had the dominion entailed on him, and Joseph the double portion, and therefore these two tribes were first seated, Judah in the southern part of the land of Canaan and Joseph in the northern part, and on them the other seven did attend, and had their respective lots as...
Commenting on Joshua 15:1-12
And the border went up by the valley of the son of Hinnom,.... Which belonged to a man of that name formerly; and was near Jerusalem, placed by Jerom (l) to the east of it; but Reland (m) rather thinks it is to the south.
The valley of the son of Hinnom - Who Hinnom was is not known, nor why this was called his valley. It was situated on the east of Jerusalem; and is often mentioned in Scripture. The image of the idol Molech appears to have been set up there; and there the idolatrous Israelites caused their sons and daughters to pass through the fire in honor of that demon, Kg2 23:10.