Grid Digits
Each of the 100 km by 100 km lettered squares are further subdivided into 100 squares - each of them 10 km by 10 km. The gridlines created from these squares are marked from 0-9 along the horizontal and vertical-hand axis, with 0,0 describing a position at the bottom left hand corner.
Measurements along the horizontal axis (running west-east) are called 'eastings' (as they run towards the east), whilst measurements along the vertical axis (from south-north) are called 'northings' (as they run towards the north).
For example, Edinburgh has a grid reference of NT 27; this means that it is situated in a square whose origin (its south-west corner) is 2 squares east of and 7 squares north of the origin of the NT square, i.e. 20 km east and 70 km north from the bottom left hand corner.