A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
neh 12:1PRIESTS AND LEVITES WHO CAME UP WITH ZERUBBABEL. (Neh 12:1-9)
these are the priests--according to Neh 12:7, "the chief of the priests," the heads of the twenty-four courses into which the priesthood was divided (1Ch. 24:1-20). Only four of the courses returned from the captivity (Neh 7:39-42; Ezr 2:36-39). But these were divided by Zerubbabel, or Jeshua, into the original number of twenty-four. Twenty-two only are enumerated here, and no more than twenty in Neh 12:12-21. The discrepancy is due to the extremely probable circumstance that two of the twenty-four courses had become extinct in Babylon; for none belonging to them are reported as having returned (Neh 12:2-5). Hattush and Maadiah may be omitted in the account of those persons' families (Neh 12:12), for these had no sons.
Ezra--This was most likely a different person from the pious and patriotic leader. If he were the same person, he would now have reached a very patriarchal age--and this longevity would doubtless be due to his eminent piety and temperance, which are greatly conducive to the prolongation of life, but, above all, to the special blessing of God, who had preserved and strengthened him for the accomplishment of the important work he was called upon to undertake in that critical period of the Church's history.
neh 12:4Abijah--one of the ancestors of John the Baptist (Luk 1:5).
neh 12:9their brethren, were over against them in the watches--that is, according to some, their stations--the places where they stood when officiating--"ward over against ward" (Neh 12:24); or, according to others, in alternate watches, in course of rotation.
neh 12:10SUCCESSION OF THE HIGH PRIESTS. (Neh. 12:10-47)
Jeshua begat Joiakim, &c.--This enumeration was of great importance, not only as establishing their individual purity of descent, but because the chronology of the Jews was henceforth to be reckoned, not as formerly by the reigns of their kings, but by the successions of their high priests.
neh 12:11Jaddua--It is an opinion entertained by many commentators that this person was the high priest whose dignified appearance, solemn manner, and splendid costume overawed and interested so strongly the proud mind of Alexander the Great; and if he were not this person (as some object that this Jaddua was not in office till a considerable period after the death of Nehemiah), it might probably be his father, called by the same name.
neh 12:12in the days of Joiakim were priests, the chief of the fathers--As there had been priests in the days of Jeshua, so in the time of Joiakim, the son and successor of Jeshua, the sons of those persons filled the priestly office in the place of their fathers, some of whom were still alive, though many were dead.
neh 12:23The sons of Levi . . . were written in the book of the chronicles--that is, the public registers in which the genealogies were kept with great regularity and exactness.
neh 12:27at the dedication of the wall of Jerusalem--This ceremony of consecrating the wall and gates of the city was an act of piety on the part of Nehemiah, not merely to thank God in a general way for having been enabled to bring the building to a happy completion, but especially because that city was the place which He had chosen. It also contained the temple which was hallowed by the manifestation of His presence, and anew set apart to His service. It was on these accounts that Jerusalem was called "the holy city," and by this public and solemn act of religious observance, after a long period of neglect and desecration, it was, as it were, restored to its rightful proprietor. The dedication consisted in a solemn ceremonial, in which the leading authorities, accompanied by the Levitical singers, summoned from all parts of the country, and by a vast concourse of people, marched in imposing procession round the city walls, and, pausing at intervals to engage in united praises, prayer, and sacrifices, supplicated the continued presence, favor, and blessing on "the holy city." "The assembly convened near Jaffa Gate, where the procession commences. Then (Neh 12:31) I brought up the princes of Judah upon the wall (near the Valley Gate), and appointed two great companies of them that gave thanks, whereof one went on the right hand upon the wall towards the dung gate (through Bethzo). And after them went Hoshaiah, and half of the princes of Judah. And (Neh 12:37) at the fountain gate, which was over against them, they (descending by the Tower of Siloam on the interior, and then reascending) went up by the stairs of the city of David, at the going up of the wall, above the house of David, even unto the water gate eastward (by the staircase of the rampart, having descended to dedicate the fountain structures). And the other company of them that gave thanks went over against them (both parties having started from the junction of the first and second walls), and I after them, and the half of the people upon the wall, from beyond the tower of the furnaces even unto the broad wall (beyond the corner gate). And from above the gate of Ephraim, and above the old gate (and the gate of Benjamin), and above the fish gate, and the tower of Hananeel, and the tower of Meah, even unto the sheep gate; and they stood still in the prison gate (or high gate, at the east end of the bridge). So stood the two companies of them that gave thanks in the house of God, and I, and half of the rulers with me (having thus performed the circuit of the investing walls), and arrived in the courts of the temple" [BARCLAY, City of the Great King].
neh 12:43the joy of Jerusalem was heard even afar off--The events of the day, viewed in connection with the now repaired and beautified state of the city, raised the popular feeling to the highest pitch of enthusiasm, and the fame of their rejoicings was spread far and near.
neh 12:44portions of the law--that is, "prescribed by the law."
for Judah rejoiced for the priests and . . . Levites that waited--The cause of this general satisfaction was either the full restoration of the temple service and the reorganized provision for the permanent support of the ministry, or it was the pious character and eminent gifts of the guardians of religion.
neh 12:45the singers and the porters kept . . . the ward of the purification--that is, took care that no unclean person was allowed to enter within the precincts of the sacred building. This was the official duty of the porters (Ch2 23:19), with whom, owing to the pressure of circumstances, it was deemed expedient that the singers should be associated as assistants.
neh 12:47all Israel . . . sanctified holy things unto the Levites,--&c. The people, selecting the tithes and first-fruits, devoted them to the use of the Levites, to whom they belonged by appointment of the law. The Levites acted in the same way with the tithes due from them to the priests. Thus all classes of the people displayed a conscientious fidelity in paying the dues to the temple and the servants of God who were appointed to minister in it.