Thomas Wilhelm, A Military Dictionary and Gazetteer
“ From the nearness of the enemy’s works, running a sap is an extremely dangerous operation. When possible, therefore, it is carried on at night; in any case, the sappers are relieved at least every hour. When a sap is enlarged to the dimensions of a trench, it bears that name. When the fire of the enemy is slack, so that many gabions may be placed and filled at the same time, it is called a flying sap. If two parapets, one on each side of the trench, be formed, it is then called a double sap. ”
