The national grid is a system of grid references used by the Ordnance Survey in their maps of the Great Britain. First, the area is divided into a number of squares, 10km by 10km.

HLHMHNHOHPJLJM
HQHRHSHTHUJQJR
HVHWHXHYHZJVJW

NANBNCNDNEOAOB
NFNGNHNJNKOFOG
NLNMNNNONPOLOM
NQNRNSNTNUOQOR
NVNWNXNYNZOVOW

SASBSCSDSETATB
SFSGSHSJSKTFTG
SLSMSNSOSPTLTM
SQSRSSSTSUTQTR
SVSWSXSYSZTVTW

Within each square, eastings and northings are given as a code. For example, HL35 means 'square HL, 3 km east, 5 km north'. Additional precision is specified by longer numbers - SS390465 means 'square SS, 3900 metres east, 4650 meters north'.