That peculiar method by which a besieger's zig-zag approaches are continuously advanced in spite of the musketry of the defenders; gabions are successively placed in position, filled, and covered with earth, by men working from behind the last completed portion of the trench, the head of which is protected by a moving defence called a sap-roller. Its progress is necessarily slow and arduous. There is also the flying sap, used at greater distances, and by night, when a line of gabions is planted and filled by a line of men working simultaneously; and the double sap, used when zig-zags are no longer efficient, consisting of two contiguous single saps, back to back, carried direct towards the place, with frequent returns, which form traverses against enfilade; the half-double sap has its reverse side less complete than the last.