Fortification Friday: Curtains on a fort? Yes, there are faces, flanks and curtains!

Last week we moved to the horizontal plane and discussed the plan or, as I prefer, the trace of a field fortification.  A trace depicts advanced and retired parts, with salient or re-entering lines between the points of the fortification.

Another way to put this, the nature of a defense requires a fort to have intersecting lines, as opposed to a single line or a set of parallel lines.  That is because a defensive line should provide the defender a means to attack the assailing body’s flanks.  In order to generate the “combat multiplier,” the defensive arrangements had to offer something better than a face-to-face battle line.  Thus the need for these intersecting lines which enabled the defender to work on a flank or two.

In regard to these intersecting lines, classified as salient or re-entering, Mahan wrote:

When such a disposition is made, it is denominated by a flanked disposition; because the enemy’s flank is attained by the fire of the retired parts when he is advancing upon the salients.

Allow me to illustrate using one Mahan’s good old Figure 2 as a base:


You see how the dark blue line of fire from the retired part hits perfectly upon the side of the advance of the “bad guys.”  From there, Mahan introduced more terms to explain the role different lines played in the defense:

The advanced parts are denominated faces; the retired parts, which protect the faces, the flanks; the retired part connecting the flanks is the curtain.

So three new terms to illustrate here.  First the face: trace_Faces

You might notice the lines denominated as faces are exactly the same as those called out as advanced parts in the earlier discussion.  While that is not always the case, it is normal.  Advanced parts face the enemy… and thus are generally faces of the fortification.

Then the flank:


Again, we see some overlap in the terms used.  Flanks are one component of the retired parts, and are the same as re-entering lines.  But the term “flank” here is referring to the purpose, while re-entering is referring to the orientation.  Function and form, if I may.

And the curtain: Trace_Curtain

The curtain is a component of the retired parts, being between two flank lines.  One might dismiss the curtain as just a necessary connecting line of lesser importance.  But the curtain played a vital role to the defense, as it formed the base line on which the rest were oriented.  What’s behind the curtain?  The very thing which the fortification is designed to defend.  The salients and all those other lines are determined so as to prevent the “bad guys” from gaining any position in front of the curtain.  So when planning, it is the curtain which is set first.

Great, we have lots of lines.  What else? We are told in geometry class that intersecting lines create angles.  And in the Mahanian instruction, those angles have names:

An angle formed by two faces is denominated a salient angle; that formed by two retired parts a re-entering angle; and one made by a face and the opposite flank, an angle of defense.

And I’ll show those here.  First the salient angle:


The salient angle is of course not the blue lines, but rather the space bordered by those lines, on the interior of the works.

Now the re-entering angle: Trace_Reentering_Angle

While in the diagram the re-entering angle appears as obtuse, that was not always the case when applied to the field.  Also note that while the diagram shows generally complementing angles – salient and re-entering – that was not always the case when the plan was adapted to the reality of the ground.  But as a general rule, one looked to form complementing angles because that gave a better definition of the third angle mentioned – the angle of defense.

The angle of defense is perhaps the most important to consider. And we have two in the figure to consider:




Notice this angle is the only one of the three defined with a line departing from the fortification walls.  It defines the angle of fires that the defender can offer from the flank lines.  Thus the angle is the primary zone in which the defender can bring resistance to bear against the attacker – a crossfire between the face and the flank lines.  This depicts, on paper, the ability of the fortification to resist a particular line of advance.

So if the officer laying out the fort can predict the most likely line of advance the enemy might select, the fortification built in response should offer an angle, or angles, of defense that directly addresses that line.

But there is an opposite to the angle of defense.  That is the dead zone or dead angle.  And that, which the engineer would hope to diminish or mitigate, is the subject of the next Fortification Friday.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 4.)


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