The first line generally consists of two parts, one an obstacle,
and is continuous except for narrow inconspicuous passages
that may serve as exits.
The supporting line should always be constructed
as a second line in the event that the first line is lost.
It may or may not be continuous—it is preferable that it should be.
There will be a certain number of points
the loss of which would seriously endanger the lines.
These must be carefully concealed for maximum effect,
and should be protected by an obstacle
which may consist of a line.
As to their number or relative position with regard to the first line,
subsequent lines should be constructed so that
the whole of the first line
can be swept with machine gun fire.