657. The object of the flank guard is to hold the enemy in check long enough to enable the main body to pass, or, like the advance guard, to enable the main body to deploy. Like all other detachments, it should be no larger than is necessary, and should not be detailed except when its protection is required. 658. When a flank guard consists of a regiment or less, its distance from the main body should not exceed a mile and a half. Practicable communication must exist between it and the main body. 659. The flank guard is marched as a separate command; that is, with advance or rear guards or both, as circumstances demand, and with patrolling on the exposed flank. 660. At times it may be necessary for an advance-guard commander to send out large reconnoitering parties which temporarily assume the character and duties of a flank guard. Such parties should be given specific orders as to when and where they are to rejoin the column. CAMPS. _SANITATION._ 661. If the area of the available ground is sufficient and suitable, the camp of the battalion or regiment should conform to the plates published in the Field Service Regulations. Under similar favorable conditions, the brigade may camp in column or in line of columns. In the latter formation the interval between regiments should be about 50 yards. When the camp site has a restricted area, intervals and distances are reduced. Under service conditions, camp sites that will permit the encampment of regiments and brigades as above indicated will not often be available and regularity must be sacrificed. 662. In large commands the halt order should assign camp sites to the next smaller commands, and the commanders of the latter should locate their respective commands to the best advantage on the area assigned them. _The Selection of Camp Sites._ 663. In campaign, tactical necessity may leave little choice in the selection of camp sites, but under any conditions the requirements of sanitation should be given every consideration consistent with the tactical situation. 664. Great care should be exercised in selecting sites. In general, the following principles govern: The site should be convenient to an abundant supply of pure water. Good roads should lead to the camp. Interior communication throughout the camp should be easy. A camp near a main road is undesirable on account of dust and noise. Wood, grass, forage, and supplies should be at hand or easily obtainable. The ground should accommodate the command without crowding and without compelling the troops of one unit to pass through the camp of another. The site should be sufficiently high and rolling to drain off storm water readily, and, if the season be hot, to catch the breeze. In cold weather it should preferably have a southern exposure, with woods to the north to break the cold winds. In warm weather an eastern exposure, with the site moderately shaded by trees, is desirable.