Chp. 3 verse 1 - chp. 4 verse 12: Introduction to Aaron and his sons that would serve as priests in the Mishkan. The command to Moshe from Hashem to designate the tribe of Levi to serve the priests in the Mishkan, as substitutes for the firstborn Israelites. By Hashem's command Moshe counts the three families of the Levites, by their individual families. He designates which direction they would camp and what their duties would be. Chp. 4 Moshe counts the family of Kehath and describes their duties.