A boundary marker, border marker, boundary stone, or border stone is a robust physical marker that identifies the start of a land boundary or the change in a boundary, especially a change in direction of a boundary. There are several other types of named border markers, known as pillars, obelisks, and corners. Border markers can also be markers through which a border line runs in a straight line to determine that border. They can also be the markers from which a border marker has been fixed.
According to Josiah Ober, boundary markers are "a way of imposing human, cultural, social meanings upon a once-undifferentiated natural environment." Boundary markers are linked to social hierarchies, since they derive their meaning from the authority of a person or group to declare the limits of a given space of land for political, social or religious reasons. Ober notes that "determining who can use parcels of arable land and for what purpose, has immediate and obvious economic implications."
Many borders were drawn along invisible lines of latitude or longitude, which often created a need to mark these borders on the ground, as accurately as possible, using the technology of the day. Advances in GPS technology have shown that there are many borders inaccurately marked on the ground.
Boundary markers have often been used to mark critical points on political boundaries, i.e. those between countries, states or local administrations, but have also been used to mark out the limits of private landholdings, especially in areas where fences or walls are impractical or unnecessary.
In developed countries the use of markers for land ownership has in many places been replaced by maps and land ownership registration. Boundary markers are not legal markers in Western countries and may have troublesome legal effects. However, boundary markers have legal meaning in Japan, and are generally installed across the country. Markers are still used extensively for marking international borders, which are traditionally classified into two categories: natural boundaries, correlating to topographical features such as rivers or mountain ranges, and artificial boundaries, which have no obvious relation to topography. The latter category includes borders defined by boundary markers such as stones and walls. International boundary markers are placed and can be maintained by mutual agreement of the bordering countries.
Boundary markers, traditionally, were often made of stone, but later many have been made with concrete or a mixture of materials. They are typically placed at a notable or especially visible point. Many are inscribed with relevant information such as the abbreviation of the boundary holder and often a date.