The former is applied to the side of France towards Belgium, and the latter, with certain modifications, to the defence of Western Germany.

The first line of fortifications on the northern frontier of France consists of Dunkirk, Lille, Valenciennes, Conde, Quesnoy, Rocroi, Charlemont, Mezieres, and Sedan; the second line, of Calais, Andres, St.Omer, Bethune, Arras, Douai, Chambrai, Landrecies, and Avesnes; the third line, of Boulogne, Montreuil, Hesdin, Abbeville, Amiens, Bapaume, Peronne, Ham, and Laon.
For mountainous frontiers it is deemed necessary to secure all the important passes with small redoubts or military works, and to defend with strong forts the grand interior strategic points on which these communications are directed.

For a frontier of moderate extent there may be some six or eight gorges in the mountains by which an army might penetrate; but it will always be found that these roads concentrate on two or three points in the great valleys below.

Take, for example, the frontier of France towards Switzerland and Italy.

The passes of the mountains are secured by the little works of Fort L'Ecluse, Fort Pierre-chatel, Fort Barraux, Briancon, Mont Dauphin, Colmars, Entrevaux, and Antibes; while Besancon, Grenoble, and Toulon, form a second line; and Lyons a grand central depot.
Where a great river or chain of lakes forms the boundary of a state, the system of defence will be much the same as that of an open land frontier, the works of the first line being made to secure the great bridges or ferries by which the enemy might effect a passage; those of the second line, to cover the passes of the highlands that generally approach more or less near the great watercourse; and those of the third line, far enough in rear to protect the great internal communications of the country.