1 Chronicles 15:11

Zadok and Abiathar the priests. Zadok may have been high priest during the later years of Saul, after Ahimelech had been slain in the slaughter of the priests of Nob and his sole surviving heir, Abiathar, became a fugitive with David, being thus cut off from contact with the tabernacle (1 Samuel 22:9-23). If Zadok was Saul’s high priest (and if he was the Zadok of 1 Chronicles 12:28, although not there described as a priest), that could account for his serving during David’s reign as joint high priest with Abiathar. In that case his serving at the tabernacle in Gibeon (1 Chronicles 16:39) may have been a continuation of his former office. See further on 2 Samuel 8:17.