Detail Showing A Section Of Continuous Tenaille Line. Fortifications Of Selma, Alabama. 1865.

Tenaille lines were, according to some engineering manuals, developed to correct the defects inherent in the redan and curtain trace, that is, give better covering fire across the salients of the line's redans and eliminate dead spaces immediately in front of the line.

Draw a right line of any length and mark the points of division along the line that locate the positions of the line's redan capitals. In this case the capitals will be 100 yards apart.

Bisect each 100 yard distance between capitals and mark the points of bisection. These points will be used to locate the re-entering angles where the faces of collateral redans intersect on a second interior boundary line. They will also divide the front boundary into line segments that are 50 yards long.

Drop perpendicular lines from each point of division on the front boundary line that are equal in length to the length of each front boundary line segment, that is, in this case, 50 yards. Connect the interior extremities of the perpendicular lines by drawing a right line from one end of the tenaille line to the other. This creates a series of square boxes: all the angles are equal and all sides are equal.

To finish the equal angle tenaille line begin at one or other end of the front boundary line and draw a diagonal line across each box, from the front boundary line to the interior extremity of the next perpendicular line. This creates the typical tenaille line lateral zig-zag pattern of alternating salient and re-entering angles.

This construction produces diagonals, which represent the faces of the redans, that are approximately 71 yards long; the interior crest of each redan would therefore have a development of 142 yards on a lateral front of 100 yards.

The length of the diagonals and therefore the number of troops required to adequately man the interior crest could be reduced by decreasing the length of the redan capitals. This also re-arranged the defensive characteristics of the line by altering the measures of both the redan salient angles and the angles of defense. Shortening the capitals to 33 yards would increase the salients and angles of defense to 114 degrees and decrease the development of the interior crest of each redan to 120 yards. This would reduce the burden of an adequate defense by 22 men, but it would also require flanking defense of collateral redan faces to be quite oblique and therefore less certain and less effective.