Line of Lunettes and Redans. From J. Laisne, Aide-Memoire Portatif. (1861 Ed.) Chapter VII, Plate 4, Figure 21.

Learning how to establish defensive relationships between detached field works can be a very difficult thing even if the conjoined principles of mutual defense and the flanked disposition are easy enough to grasp given a moment's silent reflection. Continuous lines of field works were considered, before the American Civil War, to have a tendency to inhibit an army's ability, if not necessarily its willingness, to suddenly and fortuitously jump from a strictly passive defense of a fortified position to attack the attackers when those poor souls had spent their limited energy vainly bashing themselves against a strong line of field works. To conduct an attack troops were considered to require more or less even ground that would allow them to advance rapidly and maintain their delicate formations; certainly passing over a parapet eight feet tall and crossing a ditch six or ten feet deep and 10 or twelve feet wide would, of course, prevent any offensive action at all. Armies capable of something more than pure passivity, but still so weak that they required the extra strength and confidence instilled by the protection of field works the best answer was to leave gaps in their entrenched lines that would permit troops to sally out when needed to complete the defeat of an attacking army.

If this explains the need for the development of entrenched lines with intervals between field works, it hardly explains the complexities involved in theoretical tracings of lines with intervals in which the various detached field work composing a line are all neatly arranged to both illuminate and drain every last ounce of benefit out of the principle of the flanked disposition.

Draw a right line of any length. Mark positions of the salient angles of the first line of lunettes at 300 yard intervals. Bisect each 300 yard interval and mark the points of bisection; these points will be used to locate the positions of the salient angles of the second line of redans.

From each point of bisection between the positions of the first line lunette salients, drop perpendiculars and extend them 120 yards toward the interior side of the boundary line.

Produce right lines from the points on the boundary line marking the positions of the lunette salients through the interior extremities of the perpendiculars. Extend the lines to the point where they intersect each other in rear of the positions of the lunette salients (these lines will later be used to establish the position of a third line of redans or epaulments).

Establish the faces of the second line of redans by taking a distance of 40 yards along the lines drawn in the previous step from their points of intersection with the perpendiculars dropped between the positions of the lunette salients on the boundary line.

Establish the lines of the first line's lunette faces by producing right lines from the marked positions of the lunette salients on the boundary line to the extremities of the second line redan faces. The faces of the lunettes are also given a length of 40 yards and are drawn from the lunette salients toward the interior side of the lines just completed. This tracing insures that the faces of the second line redan are perpendicular to the faces of the first line lunettes, which, of course, means that columns of fire produced from the redans will defend both the parapet and ditch of the first line lunettes during a close assault.

Flanks of the first line lunettes are established by describing arcs of circles from the extremities of the lunettes faces that have a radius of 20 yards. From the marked points of collateral lunette salients draw right lines that are tangent to the described arcs. Drop lines from the extremities of the lunette faces that are perpendicular to these tangent lines. This produces lunette flanks that are 20 yards long and whose columns of fire defend the salient angles of collateral lunettes.

To convert the second line of redans into lunette capable of defending the intervals between and ground in rear of the first line lunettes repeat the foregoing step at the extremities of the redans' faces to produce flanks that are 20 yards long. Note that the tangent line that intersects the arc centered at the extremities of the redans' faces originates from salient angles of lunettes of the first line and that the line runs to the rear of the two first line lunettes immediately to the front of second line redans.

Third line redans or epaulments for cavalry are produced by marking distances of 30 yards from the points of intersection of the lines drawn to establish the faces of the second line redans. To find the faces of the third line redans cast perpendicular lines from the 30 yard marks toward the front or engaged side of the line and extend them to their points of intersection, which will be on the same line as the capitals of the first line lunettes.