We frequently hear about the “twelve tribes of Israel” and fail to realize that’s only a class title of the group, not the actual number. If you study the Bible carefully, you’ll notice thirteen tribes exist:
The twelve tribes of Israel (Jacob) are listed twenty times in the Old Testament. They are listed by mother (Leah, Rachel, Zilhah, and Bilhah), their numeration, their encampment, order of march, their geographical relations, etc. Each time they are listed in a different order.
The Levites were exempt from military duties. When the order of military march is given, there are still 12 listed, excluding Levi. How? By dividing Joseph into two: Ephraim and Manasseh.
(Levi is thus omitted on four occasions. In a similar manner, Dan is omitted on three occasions, the most notable one in Revelation 7.)
Joseph had two sons in Egypt, Ephraim and Manasseh. Recall in Genesis Jacob claimed Joseph’s two sons as his own, making 13 actual tribes in Israel as the tribe of Joseph can be counted as Ephraim and Manasseh. Depending on which list you look at, you can see the tribe of Joseph, or Ephraim and Manasseh listed to make the twelve.
The interesting part becomes which tribe doesn’t appear in each list and why.