Grid references define locations on maps using Cartesian coordinates. Grid lines on maps define the coordinate system, and are numbered to provide a unique reference to features.
Grid systems vary, but the most common is a square grid with grid lines numbered sequentially from the origin at the bottom left of the map. The grid numbers on the east-west (horizontal) axis are called Eastings, and the grid numbers on the north-south (vertical) axis are called Northings.
Numerical grid references consist of an even number of digits. Eastings are given before Northings. Thus in a 6 digit grid reference 123456, the Easting component is 123 and the Northing component is 456.
Grids may be arbitrary, or based on specific distances, for example the United Kingdom Ordnance Survey maps use a one-kilometre square grid spacing.