The ground being previously marked out, battalions of light troops, termed _guards of the trenches_, as soon as it is dark, are placed about thirty yards in front of the first parallel, (A.Fig.

54,) with smaller sections, and sentinels about the same distance further in advance.

These guards lie down, or otherwise conceal themselves from the fire of the work.

The engineer troops and detachments of workmen being first marched to the depots and supplied with all the necessary tools for carrying on the work, now commence their labors under the protection of these guards.

By daybreak the construction of the first parallel, and the trenches connecting it with the depots, will be sufficiently advanced to cover the men from the fire of the place; the guards will therefore be withdrawn, and the workmen continue their labors during the day to give the trenches the proper size and form.
The _parallels_ are the long lines of trench which envelop the besieged work, and serve both as covered ways for the circulation of the besiegers, and as means of defence against sorties from the garrison; they are therefore arranged with banquettes for musketry fire.