What is the Ordnance Survey National Grid?

National Grid references are a compact way of stating the easting and northing coordinates of any point in Great Britain…

The National Grid is used on all Ordnance Survey maps. By giving a National Grid reference, you can pinpoint any location in Great Britain. 

The video below explains how the National Grid is devised and how to give the National Grid reference for any location:

4 figure grid references - e.g. 2000,6000

6 figure grid references - e.g. 200000,600000

Summary: How the grid reference system (British National Grid) for Great Britain works: 

Great Britain is divided into a series of 100KM X 100KM grid squares Each square has a 2 digit number and pair of letters For example:  Grid square NS is numbered 26 Top Tip: If you have trouble remembering the order, say… along the corridor, THEN up the stairs Lets imagine you want to quote a grid reference for the southwest corner of grid square NS This can be quoted in 3 ways :- NS 000,000 OR 2000,6000 (4 figure) OR 200000,600000 (6 figure) The same applies to any other grid square In summary, the southwest corner of grid square NS could be quoted as  NS 000,000 OR 2000,6000 OR 200000,600000