Small permanent works, termed redoubts, are placed within the demi-lune and re-entering places of arms for strengthening those works.  Works of this character constructed within the bastion are termed interior retrenchments; when sufficiently elevated to command the exterior ground, they are called cavaliers.

Caponniers are works constructed to cover the passage of the ditch from the tenaille to the gorge of the demi-lune, and also from the demi-lune to the covered way, by which communication may be maintained between the enceinte and outworks.

Posterns are underground communications made through the body of the place or some of the outworks.

Sortie-passages are narrow openings made through the crest of the glacis, which usually rise in the form of a ramp from the covered way, by means of which communication may be kept up with the exterior.  These passages are so arranged that they cannot be swept by the fire of the enemy.  The other communications above ground are called ramps, stairs, &c.

Traverses are small works erected on the covered way to intercept the fire of the besieger’s batteries.

Scarp and counterscarp galleries are sometimes constructed for the defence of the ditch.  They are arranged with loop-holes, through which the troops of the garrison fire on the besiegers when they have entered the ditch, without being themselves exposed to the batteries of the enemy.

In sea-coast defences, and sometimes in a land front for the defence of the ditch, embrasures are made in the scarp wall for the fire of artillery; the whole being protected from shells by a bomb-proof covering over head:  this arrangement is termed a casemate.

Sometimes double ramparts and parapets are formed, so that the interior one shall fire over the more advanced; the latter in this case is called a faussebraie.

If the inner work be separated from the other it is called a retrenchment[44] and if in addition it has a commanding fire, it is termed, as was just remarked, a cavalier.

[Footnote 44:  The term retrenchment implies an interior work, which is constructed within or in rear of another, for the purpose of strengthening it; the term intrenchment, on the contrary, implies an independent work, constructed in the open field, without reference to any other adjoining work.]

The capital of a bastion is a line bisecting its salient angle.  All the works comprehended between the capitals of two adjacent bastions is termed a front:  it is taken as the unit in permanent fortification.

Fig. 39 represents the ground plan of a modern bastioned front, of a regular and simple form, on a horizontal site.