1 Kings 22:42

He reigned twenty and five years His son and successor, Jehoram, was made king in conjunction with his father, before Jehoshaphat's death (2 Kings 9:16). But this could only have been done just at the close of Jehoshaphat's reign. For it was in the fourth year of Ahab that Jehoshaphat began to reign. Ahab reigned 22 years (1 Kings 16:29). So that 18 years of Jehoshaphat's reign were over when Ahab died. Ahaziah reigned two years (see 1 Kings 22:51 below) and it was in the 5th year of Joram, the brother and successor of Ahaziah, that Jehoshaphat joined his son with him in the kingdom. So that, unless the years are not complete years, it must have been in the closing years of his father's reign that Jehoram began his joint reign.

his mother's name On the important position occupied by the queen-mother in Oriental kingdoms, see on 1 Kings 2:19. This accounts for the constant mention of her name at each king's accession.