In traversing a double sap by change of direction, a single sap is broken out to the right or left (or one in each direction) and pushed forward to the desired length. From the flank of this the double sap is again broken out and driven to the front until another traverse is required. When the saps are driven to both right and left a double sap is driven to the front from the extremity of each, and at the next change of direction the single saps are driven towards each other until they meet, and the double sap is driven in the prolongation of its original direction. This forms what is called a _cube traverse_, and gives additional room in the communications. The single sap is used in making traverses, since by throwing all the earth on one side better cover is given. When the sap is so far advanced that it becomes subject to a reverse fire the double sap will have to be used in making the traverses. _Length of Traverses._--Traverses should extend at least 12 feet beyond the trench in their rear, which will give them a length of from 25 to 30 feet on the berm. The salient angles of the sides of the trenches should be rounded as much as practicable to allow the easy passage of guns, and those of the parapets should, when necessary to screen the trench, be held as nearly vertical as practicable by sand-bags. Ramps leading to the surface of the ground may be made in rear of the traverses when needed. For guns they are 8' wide, with slope not greater than 1/4. The work in making traverses being considerable, they should be spaced as far apart as practicable. _Spacing Traverses._--In driving the sap to the front the low head parapet of the sap will defilade a less length in the rear than would the finished parapet of the traverse, which is from 2 to 3 feet higher. The sap is, nevertheless, pushed forward to as great a length as will be defiladed by the traverse when finished, the sappers meanwhile passing the partly protected portion by stooping or creeping when necessary. =35.= =Traverse by Blinded Sap.=--In traversing a sap by blinding a part of its length (Pl. III, Figs. 32-35) the sap is first deepened 2 feet over this part; mine-cases, frames similar to mining-frames, or regular blindage-frames (see Military Mining, Arts. 53-55) are then put in position, the side slopes are held up by sheeting, when necessary, and the top is covered with sheeting, fascines, rails, or other material; earth is then thrown upon the top to bring it up to the desired height for a traverse, which will usually give at least 3 feet of earth covering. When a considerable thickness of earth is to be used the frames must be made correspondingly strong. For a clear opening, 6 feet at bottom, 8 feet at top, and 6' 6" high the English engineers recommend frames at 3-foot intervals, with 6-inch square posts, 2-inch thick sills, and 9 inch × 6 inch caps 12 feet long. The end frames should be braced against outward thrust by 6" × 6" struts.