Front of a Line of Bastions with Double Flanks  from J. S. Macaulay, Treatise on Field Fortification. (1834)

A bastion line with double flanks was designed to do an even more thorough job of laying crossing columns of fire over ground immediately in front of the entrenchment while throwing a good part of the line into deep re-enterings that were not supposed to be vulnerable to assault. Greater advantages in the traces of field fortifications were almost always accompanied by greater expenses in laying them out on the ground, constructing them, and, most importantly garrisoning and defending them. If this trace improved on a regular bastion line, its realization on the ground would have been much more costly and it is a general principle of fortification that constructed fortifications should not overwhelm the defender's means of adequately defending them.

Draw a right line of any length. Each bastion front of fortification in this trace will be 400 yards long. Although this seems an excessive length for a bastion front primarily defended by small arms fire, the double flanks, which are, in effect, retired bastions in place of the straight curtains of a regular bastion front, produce sufficient flanking fire to cover the faces and flanked angles of the advanced bastions.

Bisect each front of fortification into two 200 yard segments. Drop a perpendicular line from each point of bisection that is equal in length to one-eight the total length of the front of fortification. In this case the perpendiculars are 1/8 of 400 yards or 50 yards long.

Being careful to preserve the 50 yard mark on each perpendicular, extend each perpendicular so that its overall length is equal to one-third the length of the front of fortification. In this case 1/3 of 400 is rounded to 133 yards. Draw right lines from the extremities of each front of fortification through the interior extremities of the perpendiculars; this produces the bastions' lines of defense. To establish the bastion faces draw right lines from the extremities of each front along the lines of defense that are one-fifth the length of the front of fortification. In this example 1/5 of 400 is 80 yards; so the bastion faces will be 80 yards long.

To produce the faces of the retired bastions draw right lines from the extremities of each front of fortification through the 50 yard mark each perpendicular. These lines may be considered second lines of defense. In this trace the faces of the retired bastions will be 40 yards long and are established by taking 40 yards from the point where the second lines of defense intersect the 50 yard mark on each perpendicular.

These second lines of defense are also used to establish the position and orientation of the advanced bastions' flanks while the first lines of defense are used to establish the positions of the retired bastions' flanks. Draw the advanced bastion flanks by dropping lines perpendicular to the second lines of defense from the extremities of the advanced bastions' faces; execute the flanks for the retired bastions by dropping lines from the extremities of the retired bastion faces to the first lines of defense. In this trace the advanced bastion flanks will be about 37 yards long while the retired bastions' flanks will be about 40 yards long.

This trace is completed by drawing straight curtains from the extremities of the advanced bastions' flanks to the extremities of the retired bastions' flanks. On a lateral front of 400 yards this trace develops an interior crest 524 yards which would require 1048 men in two ranks to adequately defend. Compared to the various tenaille traces, the bastion line with double flanks does not seem to require an excessive number of defenders.