The National Grid

"The original 'Romer' was invented by Lieutenant Carrol Romer of the First Army’s Maps and Printing Section of the Royal Engineers in the First World War. The Lieutenant, who was a barrister in his civilian days, was born in 1883 and died in 1951. His invention, printed on cardboard, was first used on the Army’s 1:40 000 scale maps of the Northern France battlefields. The device was first referred to as ‘a romer’ in the Geographical Journal of 1933."

In the British Isles we use Ordnance Survey Landranger maps of which there are 204 in total.
Each map covers an area of 25 x 25 miles (40 x 40 Km.) A series of vertical and horizontal numbered lines will be found on these maps. Called grid lines they form part of the National Grid which is based on an imaginary line to the West of Lands End. Further to this, each 10x10Km area has a two letter prefix, which enables a uniqueness when plotting a map reference.

The National Grid:
 On all maps produced by the Ordnance Survey, there was a need to print some form of grid lines on these maps so that any place could easily be referred to. Initially it was thought that lines of latitude and longitude would be adequate, but there is a problem with this.  
Latitude is not so bad because lines of latitude are exactly parallel to each other and almost equidistant from each other, one degree representing 110567 metres at the equator increasing gradually to 111699 metres at the North Pole.
Longitude is the problem, because a degree of longitude at the equator is 113200 metres long decreasing to 0 metres at the North Pole!
Even in our small country the difference is significant: In Cornwall one degree of longitude is 71700 metres long and in the North of Scotland it is only 54300 metres.

In other words, whilst the lines of latitude are parallel with each other, the lines of longitude curve inwards as you move north, because they all converge to a single point at the North Pole  (=True North) 

It was clear then that the OS needed a completely separate grid system which consisted of accurate and consistent squares right across the whole country, with all the horizontal lines parallel and all the vertical lines parallel.  

The new system, called ‘The National Grid’ came about as a result of a report by Sir John Davidson in 1935 who was given responsibility for accurately re-mapping the country for military purposes, as war with Germany seemed to be ever more likely.

The origin of the new grid was set at a position just south west of the Isles of Scilly. It was also agreed that the scale of the grid would be Metric, i.e. the grid would be based on one Kilometre squares (even on the 1” to a mile maps then in use).
It was decided that the whole grid would be aligned to the 2º W longitude line which runs roughly down the middle of the country (through Walsall and West Bromwich in fact). 

A full reference (in metres) is always possible with the National Grid, without reference to any particular map number or scale.

The simple reference 822 991 on its own occurs every 100 kilometres, in other words there are many places in the UK that have 822 991 as their 6-figure reference for instance.
The Ordnance Survey allocated a letter code to each 100 km square.

The OS would prefer us to refer 822 991 as SO/822991 because that is totally unique across the country irrespective of what scale or type of map is being used.