Artillery fire is effective in proportion to its concentration. The long range of the guns permits a concentration of their fire on many different points without change of position. When practicable, the groups that are intended to fire on the same target should be placed under the control of one officer. In the advance-guard action the artillery of the assailant covers, and the artillery of the defence opposes, the deployment and advance of the advance-guard infantry. In the preliminary stages of a battle and the commencement of the artillery duel the fire is usually directed on the targets immediately opposite the batteries engaged; as soon as portions of the enemy's artillery-line are subdued the fire is concentrated on the different targets in succession, taking them in the order of their relative importance; during the preparation and delivery of the assault the fire of the assailants is concentrated against the point of attack; the fire of the defence is concentrated against the attacking infantry as soon as it shows itself. Fire should be directed against batteries that are changing positions, and in order that this brief opportunity of inflicting serious damage may not be lost a part of the batteries of the defence should be especially instructed to open fire on hostile batteries while limbering or in movement without waiting for orders. The defence generally offers more favorable opportunities for long-range fire than the offence, but whether advantage is to be taken of these opportunities must be determined by the general commanding, who will have to decide whether the effect of the fire will compensate for the partial or complete disclosure of the position that it involves. Ordinarily artillery should avoid opening fire at a greater range than 3000 yards. Beyond this distance it is difficult to observe the effect of fire even with good glasses. Longer range fire is used in exceptional cases against objects that are clearly seen and are of considerable extent, such as villages, camps, or large bodies of troops. Ineffective cannonades at long distances, and shelling woods or other localities not positively known to be occupied by the enemy, should not be permitted. The position from which fire is first opened may necessarily be much less than 3000 yards, in order to obtain view of the enemy.