638. The march order of the whole command should explain the situation, and, among other things, detail the commander and troops for each covering detachment. It should specify the route to be taken and the distance to be maintained between the main body and its covering detachments. It should order such reconnaissance as the commander specially desires to have made. The order of the commander of a covering detachment should clearly explain the situation to subordinates, assign the troops to the subdivisions, prescribe their distances, and order such special reconnaissance as may be deemed necessary in the beginning. An advance or flank guard commander marches well to the front and, from time to time, orders such additional reconnaissance or makes such changes in his dispositions as the circumstances of the case demand. _Advance Guards._ 639. An _advance guard_ is a detachment of the main body which precedes and covers it on the march. 640. The advance guard commander is responsible for its formation and conduct. He should bear in mind that its purpose is to facilitate and protect the march of the main body. Its own security must be effected by proper dispositions and reconnaissance, not by timid or cautious advance. It must advance at normal gait and search aggressively for information of the enemy. Its action when the enemy attempts to block it with a large force depends upon the situation and plans of the commander of the troops. 641. The strength of the advance guard varies from one-twentieth to one-third of the main body, depending upon the size of the main body and the service expected of the advance guard. 642. The formation of the advance guard must be such that the enemy will be met first by a patrol, then in turn by one or more larger detachments, each capable of holding the enemy until the next in rear has time to deploy before coming under effective fire. 643. Generally an advance guard consisting of a battalion or more is divided primarily into the _reserve_ and the _support_. When the advance guard consists of less than a battalion, the reserve is generally omitted. 644. In an advance guard consisting of two battalions or less, the reserve and support, if both are used, are approximately equal; in larger advance guards, the reserve is approximately two-thirds of the whole detachment. In an advance guard consisting of one battalion, the machine guns, if any, form part of the reserve. In an advance guard consisting of two or more battalions, the machine guns form part of the support.