An advance outpost or guard for a large force was called a picket. Ordered to form a scattered line far in advance of the main army's encampment, but within supporting distance, a picket guard was made up of a lieutenant, 2 sergeants, 4 corporals, and 40 privates from each regiment. Picket duty constituted the most hazardous work of infantrymen in the field. Being the first to feel any major enemy movement, they were also the first liable to be killed, wounded, or captured. And he most likely targets of snipers. Picket duty, by regulation, was rotated regularly in a regiment.
Soldiers posted on guard ahead of a main force. Pickets included about 40 or 50 men each. Several pickets would form a rough line in front of the main army's camp. In case of enemy attack, the pickets usually would have time to warn the rest of the force.