the basic front line fortifications were open or covered machine gun nests, rifle pits, observation posts and shelters. most of the structures were designed to be covered with a concrete roof poured on wooden planks occasionally reinforced with railway tracks. some of the positions were built with stone or logs. many of the fortifications have been left uncovered however, built only to a point where adding the missing roof could be accomplished quickly. the basic fortified types are a type a square machine gun nest, type b square rifle or machine gun pit, type c rounded rifle or observation pit, type d oval rifle pit with several firing ports and types e and f large rifle positions for multiple shooters. there is considerable variation between individual structures among the basic types, with newer positions generally built larger and stronger.