How to use the Ordnance Survey National Grid Reference Numbers
 The United Kingdom is divided into a number of 100 km squares. Each square is given a unique identifier in the form of a pair of letters. If you look on the cover of an OS map, you should find which of these squares is wholly or partially covered by the map. Each of these squares is further divided into 10000 squares of 1000 metres by 1000 metres. Each of these smaller squares is referred to by the co-ordinates of its south-west corner. The co-ordinates are numbered from "00" to "99" along the bottom of the map, ("Eastings"), and from "00" to "99" up the right-hand side, ("Northings"). Each of these squares can be further divided into 100 even smaller squares of 100 metres by 100 metres. These are referred to by another series of co-ordinates, this time numbered from "0" to "9". Now that you are thoroughly confused, let us try to find our "Castle X".
 Castle X has a Grid Reference Number of "AB 982035", and a nearby church has "AB 016974". So first of all we need a map which covers the 100 km by 100 km square known as "AB", (which doesn't actually exist, I hasten to add). It would look like this [Figure 1 on the right]: Figure 1
 Using the first co-ordinates in the order "Eastings" then "Northings", (or "Along the corridor and up the stairs") we can find the two squares containing the church and Castle X. Now we zoom into that first set of co-ordinates [Figure 2 on the right]: The church ("AB 016974"), is somewhere in the RED square, while Castle X ("AB 982035"), is somewhere in the BLUE square. The trouble is that each square is 1000 metres by 1000 metres. So let's get closer. Figure 2
 The RED square first ("AB 016974") [Figure 3 on the right]: Use the extra co-ordinates of "Easting 6" and "Northing 4" and we are within 100 metres of the church. Figure 3
 Next the BLUE square "AB 982035" [Figure 4 on the right]: Use the extra co-ordinates of "Easting 2" and "Northing 5" and we're in the moat of Castle X. Easy, yes ? Figure 4