Now Moses kept the flock of Jethro his father in law, the priest of Midian: and he led the flock to the backside of the desert, and came to the mountain of God, even to Horeb. Jethro his father-in-law - Concerning Jethro, see Clarke's note on Exodus 2:18. Learned men are not agreed on the signification of the word חתן chothen, which we translate father-in-law, and which in Genesis 19:14, we translate son-in-law. It seems to be a general term for a relative by marriage, and the connection only in which it stands can determine its precise meaning. It is very possible that Reuel was now dead, it being forty years since Moses came to Midian; that Jethro was his son, and had succeeded him in his office of prince and priest of Midian; that Zipporah was the sister of Jethro; and that consequently the word חתן chothen should be translated brother-in-law in this place: as we learn from Genesis 34:9, Deuteronomy 7:3, Joshua 23:12, and other places, that it simply signifies to contract affinity by marriage. If this conjecture be right, we may well suppose that, Reuel being dead, Moses was continued by his brother-in-law Jethro in the same employment he had under his father.
Mountain of God - Sometimes named Horeb, at other times Sinai. The mountain itself had two peaks; one was called Horeb, the other Sinai. Horeb was probably the primitive name of the mountain, which was afterwards called the mountain of God, because God appeared upon it to Moses; and Mount Sinai, סיני, from סנה seneh, a bush, because it was in a bush or bramble, in a flame of fire, that this appearance was made.