The British National Grid (also called the Ordnance Survey National Grid) is a coordinate system based on the Transverse Mercator projection and OSGB36 datum [1][2]. It divides Great Britain into a hierarchical grid structure starting with 500 km squares, each identified by a first letter, then subdivided into 100 km squares identified by a second letter [2][3].

The standard format consists of a two-letter code followed by easting and northing values as digits [4][2][3]. The easting (distance east from the southwest corner) always comes before the northing (distance north) [2][3]. Both the easting and northing must contain an equal number of digits, and they are often written together without spaces, though spacing is optional [4][2].

Precision scales with digit count: a two-digit pair (e.g., NT 65 20) gives 10 km precision; four digits (e.g., NT 1234 5678) gives 1 km precision; six digits gives 100 m precision; eight digits gives 10 m precision; and ten digits gives 1 m precision [4][2]. A six-figure grid reference like NN166712 means the first three digits (166) are the easting and the last three (712) are the northing within square NN, identifying a 100 m square [4][3]. The same location can also be expressed as full numerical eastings and northings in metres [4][2].

Optionally, quadrant designations (NW, NE, SW, SE) may be added [4]. This system is the standard used by hikers, emergency services, and the Ordnance Survey itself for navigation and mapping across England, Scotland, and Wales [2][3].