PARALLELS, in siegecraft, a term used to express the trenches drawn by besiegers in a generally parallel direction to the front of a fortress chosen for attack. Parallels are employed along with " zigzag approaches " in the " formal attack " or siege proper. They are traced in short zigzag lengths (the prolongation of each length falling clear of the hostile works), in order to avoid enfilade; but their obliquity is of course made as slight as is consistent with due protection in order to save time and labour. The " first parallel " is opened at a convenient distance from the fortress, by numerous working parties, who dig (under cover of night) a continuous line of entrenchments facing the point or points of attack. Zigzags are next dug to the rear (-when necessary) to give sheltered access to the parallel, and from this new zigzags are pushed out towards the defenders, to be connected by a " second parallel," and so on until finally a parallel is made sufficiently close to the fortress to permit of an assault over the open, the parallels becoming stronger and more solid as they approach to closer range. This system of parallels provides, within range of the defenders' -ft'eapons, shelter in which the besieger can safely mass men and material for the prosecution of the attack. Parallels and approaches are constructed either by ordinary " trench work," executed simultaneously by a large number of men strung out along the intended line, or by " sapping " in which one trained " sapper," as it were, burrows a trench in the required direction, others following him to widen and improve the work.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)