Jamieson-Fausset-Brown Bible Commentary

when they shall blow with them--There seem to have been signals made by a difference in the loudness and variety in the notes, suited for different occasions, and which the Israelites learned to distinguish. A simple uniform sound by both trumpets summoned a general assembly of the people; the blast of a single trumpet convoked the princes to consult on public affairs; notes of some other kind were made to sound an alarm, whether for journeying or for war. One alarm was the recognized signal for the eastern division of the camp (the tribes of Judah, Issachar, and Zebulun) to march; two alarms gave the signal for the southern to move; and, though it is not in our present Hebrew text, the Septuagint has, that on three alarms being sounded, those on the west; while on four blasts, those on the north decamped. Thus the greatest order and discipline were established in the Israelitish camp--no military march could be better regulated.