A linear boundary (prehistoric) is a man-made feature comprising single or multiple ditches and banks which continue for distances varying between less than 1km and over 10km. In profile the ditches are either V- or U-shaped, while their dimensions range between 1.5m to 6m wide and between 0.4 to 2.0m deep. The banks are of simple dump construction and where they survive rarely exceed a height of 0.5m or a width of 3.0m. Prehistoric linear boundaries may be identified in the field as low earthworks, although in many cases these have been ploughed out and the monuments have no, or only slight, visible surface traces; they may be identified, however, from crop or soil marks visible on aerial photographs; these may be used to trace their alignment and link separate elements within a system of boundaries.
Prehistoric linear boundaries may be confused with linear boundaries of a later date; early medieval frontier works, such as Wansdyke, usually comprise more massive earthworks and may thus be distinguished. Medieval dykes built to surround parks and areas of woodland, and to separate rough grazing from better land, or divide open fields from pasture are harder to date from external appearances; they may, however, be distinguished by using historical documentary evidence. Artefactual and stratigraphic information resulting from excavation may be necessary to confirm the prehistoric origin.
Excluded from this description are prehistoric boundaries comprising pit alignments; these are treated as a separate monument class; also excluded are reaves which are treated as components of field systems. Cross-ridge dykes are also excluded.
Prehistoric linear boundaries, dating from the middle Bronze Age and throughout the Iron Age, are thought to represent territorial boundaries in an economy which included much pastoralism; the boundaries of multiple ditches and banks may mark out territories of particular importance or prestige.