If you are trying to find a location on an Ordnance Survey map, you will often find something like this:
TQ 336 805 (that’s the Tower of London, in case you were wondering)
Ordnance Survey divides Great Britain into 100 km by 100 km squares, each with a twoletter code. The twoletter codes can be found printed in faintblue capitals on Ordnance Survey maps and can also be found in the map key. TQ, in this case, covers most of London.
National Grid reference numbers
The numbers now give us a more precise location on the map. There are two sets of numbers that relate to the numbers you will find along the edges of the map.
The numbers going across the map from left to right are called eastings, and go up in value eastwards, and the numbers going up the map from bottom to top are called northings, because they go up in a northward direction.
When giving a reference, you should always give the eastings number first and the northings number second, very much like when giving the reading of a graph in school, where you give the x (along) coordinate first followed by the y (up).
An easy way to remember this is that to get the first number, you go along the corridor (horizontal, x axis, eastings) and then up the stairs (vertical, y axis, northings).
To get the sixfigure grid reference, you have to imagine that the single larger square you can get from the map grid is further divided up into tenths.
In the example below, the grey box is in the fourfigure grid reference square ‘18 44’, but more accurately it is 7 tenths across and 8 tenths up within that larger grid square, therefore the sixfigure map reference is ‘187 448’.
The shapes on the diagram above have the following sixfigure grid references:
For example, you may be at grid reference ‘509 582’ in southwest Scotland. The complete grid reference you should quote would be ‘NX 509 582’ (without the letters the numeric reference would be repeated in every 100 km square).
You may come across the abbreviated version of the terms ‘latlong’. Latitude and longitude references look like this: ‘50.855226 1.402772’ and they are quite different to the grid references commonly used for determining points on an Ordnance Survey map. You can convert between these different systems using a converter  there's one on the Ordnance Survey website.
TQ 336 805 (that’s the Tower of London, in case you were wondering)
Ordnance Survey divides Great Britain into 100 km by 100 km squares, each with a twoletter code. The twoletter codes can be found printed in faintblue capitals on Ordnance Survey maps and can also be found in the map key. TQ, in this case, covers most of London.
National Grid reference numbers
The numbers now give us a more precise location on the map. There are two sets of numbers that relate to the numbers you will find along the edges of the map.
The numbers going across the map from left to right are called eastings, and go up in value eastwards, and the numbers going up the map from bottom to top are called northings, because they go up in a northward direction.
When giving a reference, you should always give the eastings number first and the northings number second, very much like when giving the reading of a graph in school, where you give the x (along) coordinate first followed by the y (up).
An easy way to remember this is that to get the first number, you go along the corridor (horizontal, x axis, eastings) and then up the stairs (vertical, y axis, northings).
To get the sixfigure grid reference, you have to imagine that the single larger square you can get from the map grid is further divided up into tenths.
In the example below, the grey box is in the fourfigure grid reference square ‘18 44’, but more accurately it is 7 tenths across and 8 tenths up within that larger grid square, therefore the sixfigure map reference is ‘187 448’.
The shapes on the diagram above have the following sixfigure grid references:
 Grey square – 187 448
 Red dot – 185 443
For example, you may be at grid reference ‘509 582’ in southwest Scotland. The complete grid reference you should quote would be ‘NX 509 582’ (without the letters the numeric reference would be repeated in every 100 km square).
Variations of grid references
In addition to the standard four, six and tendigit formats, you may also come across several other ways that National Grid references can be quoted. For example, the summit of Snowdon’s grid reference is ‘SH 609 543’, but this may also be referred to as: An abbreviated number where the two National Grid letters are omitted, usually used if the context of the grid reference is obvious – ‘609 543’ or ‘609543 on OS Landranger Map 115’. People may also use this style to quote a reference for a place that is local to you.
 The full tendigit grid reference with National Grid letter – ‘SH 60989 54379’ or without letters ‘60989 54379’ – the most accurate grid reference. Note that these longer grid references are not that useful at popular leisure map scales.
 With preceding numbers identifying the Ordnance Survey map sheet number – ‘17/609543’ for OS Explorer Map OL17 or ‘115/609543’ for OS Landranger Map sheet 115.

Latitude and longitude
Using latitude and longitude is a more accurate method of pinpointing the exact location of a very specific place on the earth’s surface and is commonly used by satellite positioning systems and GPS devices. Latitude specifies the northsouth position of a point and longitude the eastwest position.You may come across the abbreviated version of the terms ‘latlong’. Latitude and longitude references look like this: ‘50.855226 1.402772’ and they are quite different to the grid references commonly used for determining points on an Ordnance Survey map. You can convert between these different systems using a converter  there's one on the Ordnance Survey website.