580. The detail of a support is not necessary except when the artillery is separated from the main body or occupies a position in which its flanks are not protected. The detail of a special support will be avoided whenever possible. 581. The formation of an artillery support depends upon circumstances. On the march it may often be necessary to provide advance, flank, and rear protection. The country must be thoroughly reconnoitered by patrols within long rifle range. 582. In action, the formation and location of the support must be such as to gain and give timely information of the enemy's approach and to offer actual resistance to the enemy beyond effective rifle range of the artillery's flanks. It should not be close enough to the artillery to suffer from fire directed at the artillery. In most cases a position somewhat to the flank and rear best fulfills these conditions. 583. The support commander is charged only with the protection of the artillery. The tactical employment of each arm rests with its commander. The two should cooperate. _INTRENCHMENTS._ (Plate V.) 584. Ordinarily, infantry intrenches itself whenever it is compelled to halt for a considerable time in the presence of the enemy. Infantry charged with a resisting mission should intrench whenever there is any likelihood that the cover constructed will be of use. 585. Except in permanent fortifications or in fortifications prepared long in advance, the infantry plans and constructs the field works that it will occupy. When performing their duties in this connection officers should bear in mind that profile and construction are simple matters compared with location and correct tactical use. 586. Intrenchments enable the commander to hold a position with the least possible number of men and to prolong his line or increase his reserve. They are constructed with a view to giving cover which will diminish losses, but they must not be so built or placed as to interfere with the free use of the rifle. Fire effect is the first consideration. 587. The trace of a fire trench or of a system of fire trenches depends upon the ground and the proposed density of the entire firing line. The trenches are laid out in company lengths, if possible. Adjoining trenches should afford each other mutual support. The flanks and important gaps in the line should be protected by fire trenches echeloned in rear. (Fig. 6). 588. To locate the trace, lie on the ground at intervals and select the best field of fire consistent with the requirements of the situation. A profile should be selected which will permit the fire to sweep the foreground, require the minimum of labor and time, and permit the best concealment. No fixed type can be prescribed. The type must be selected with due regard to the terrain, the enemy, time, tools, materials, soil, etc.