This arrangement constitutes the _nightly cordon_.
By the time the main army arrives the reconnoissance will be sufficiently complete to enable the chief engineer to lay before the general the outline of his plan of attack, so as to establish the position of his depots and camp.

These will be placed some two miles from the work, according to the nature of the ground.

As they occupy a considerable extent of ground around the work, it will generally be necessary to form intrenchments strong enough to prevent succors of troops, provisions, &c., from being thrown into the place, and also to restrain the excursions of the garrison.

The works thrown up between the camp and besieged place are termed the _line of countervallation_, and those on the exterior side of the camp form the _line of circumvallation_.

These lines are generally about six hundred yards apart.