Boundary markers are integral to boundary law in the United States, both in the original colonial states and those added later during westward expansion (otherwise known as the Public Land Survey System). Man-made boundary markers, or monuments, are considered to be second-highest in the Order of Evidence in boundary law in the United States, behind only natural markers such as boulders and rivers. Boundary markers also 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.