The divisions of the descendants of Aaron were as follows: Aaron's sons were Nadab, Abihu, Eleazar, and Ithamar.
References for 1 Chronicles 24:1
But Nadab and Abihu died before their father, and they had no sons, so Eleazar and Ithamar served as priests.
Together with Zadok from the sons of Eleazar and Ahimelech from the sons of Ithamar, David divided them according to the assigned duties of their service.
Since more leaders were found among Eleazar's descendants than Ithamar's, they were divided [accordingly]: 16 heads of ancestral houses were from Eleazar's descendants, and eight [heads] of ancestral houses were from Ithamar's.
They were divided impartially by lot, for there were officers of the sanctuary and officers of God among both Eleazar's and Ithamar's descendants.
The secretary, Shemaiah son of Nethanel, a Levite, recorded them in the presence of the king and the officers, Zadok the priest, Ahimelech son of Abiathar, and the heads of families of the priests and the Levites. One ancestral house was taken for Eleazar, and then one for Ithamar.
The first lot fell to Jehoiarib, the second to Jedaiah,
the third to Harim, the fourth to Seorim,
the fifth to Malchijah, the sixth to Mijamin,
the seventh to Hakkoz, the eighth to Abijah,
the ninth to Jeshua, the tenth to Shecaniah,
the eleventh to Eliashib, the twelfth to Jakim,
the thirteenth to Huppah, the fourteenth to Jeshebeab,
the fifteenth to Bilgah, the sixteenth to Immer,
the seventeenth to Hezir, the eighteenth to Happizzez,
the nineteenth to Pethahiah, the twentieth to Jehezkel,
the twenty-first to Jachin, the twenty-second to Gamul,
the twenty-third to Delaiah, and the twenty-fourth to Maaziah.
These had their assigned duties for service when they entered the Lord's temple, according to their regulations, which they received from their ancestor Aaron, as the Lord God of Israel had commanded him.