Grid references look like code. Six digits and a pair of letters that turn "somewhere near the reservoir" into a point you could walk to without hesitating — or, more usefully, a point you can hand to someone else and have them find it. The system behind those digits is simple, and after a handful of readings on paper you'll stop thinking about it.

Here is the whole thing, from the letters at the front to the last digit at the back.

What the letters are for

Ordnance Survey maps of Great Britain use the National Grid, a set of squares laid over the country. The country is divided into 100 km squares, each with a two-letter code. TQ covers London, Sussex and the Kent coast. SU takes in Wiltshire, Hampshire and Dorset. NT covers Edinburgh and the Northumberland coast. Those letters appear in the map key and along the edges of the sheet.

Inside each square, a position is given as two numbers: an easting and a northing. That's all a grid reference is.

Eastings and northings, and which comes first

Eastings are the vertical lines. They're numbered from west to east, so the number rises as you move right across the map. Northings are the horizontal lines, numbered from south to north, rising as you move up the page.

The order matters: eastings first, then northings. E before N is one way to hold on to it. The one that tends to stick, though, is "along the corridor, then up the stairs" — go right first, then climb. Reverse them and you'll land a long way from where you meant to be, which is the sort of mistake that shows up on a hillside at dusk rather than at the kitchen table.

One small thing worth knowing: the numbers printed along the edge of a map are often shortened. On a Landranger sheet you might see 31 and 79 where the full easting and northing are 531 and 179. The first digit is the same across the whole sheet, so it's left off. The reference you take from those edge labels is the one everyone else will understand.

Four, six and eight figures

The number of digits tells you how precise the reference is. More digits, smaller square.

• Four figures — TQ 31 79. A 1 km square. Fine for "the wood on the north side of the village".
• Six figures — TQ 312 794. A 100 m square. The standard for walking, meeting points and mountain rescue.
• Eight figures — TQ 3124 7946. A 10 m square. Used for search work, cave entrances and archaeology, where a few metres matter.

You'll see references written without the letters, or with a different spacing. The safe format is letters, then the two numbers, each split in the middle: TQ 312 794.

Taking a six-figure reference, step by step

1. Find the two-letter square code for your sheet, from the key or the margins.
2. Put your finger on the point you want. Find the vertical grid line immediately to the left of it and read that line's number from the top or bottom edge of the map. Those are the first two easting digits.
3. Judge how far your point sits between that line and the next one to the right. A tenths estimate gives you the third digit — halfway across is 5.
4. Do the same with the horizontal lines. Find the line immediately below your point and read its number from the left or right edge. First two northing digits.
5. Estimate tenths upwards to your point for the final digit.
6. Write it as letters, easting, northing: TQ 312 794. You've just described a 100 m square.

Reading the tenths by eye

Splitting a grid square into ten is the only fiddly part, and it comes quickly with practice. Halve the square, halve again for quarters, then decide which tenth you're in. Use whatever the map gives you: a stream, a field boundary, the bend in a lane. If a feature sits right on a grid line, your easting ends in 0.

It helps to know what 100 m looks like on the ground — roughly the length of a football pitch. An error of one tenth is an error of about 100 m, which is the difference between the right gate and the wrong one.

Where beginners go wrong

• Northing first. The most common slip of all. Say "along the corridor" every time.
• Using the nearest line instead of the one to the left and below. The digits describe the lower-left corner of the square, not the point itself.
• Dropping the letters. Six digits on their own mean nothing outside the sheet they came from.
• Claiming more accuracy than the map supports. A 1:50,000 map won't reliably give you 10 m readings, however neat your eight figures look.
• Mixing this up with latitude and longitude, or with the Irish Grid used in Northern Ireland, which has its own lettering. They don't interchange.

Putting it to use

Practise on a landmark you can see. Take a six-figure reference for it, walk there, and check whether you end up in the right spot. Then reverse it: give the reference to someone else and let them find you. That's the whole skill, and it takes an afternoon to bed in.

Most mapping apps, including OS Maps, will show the grid reference for a point you tap or long-press. That makes a useful check on your own eye — do a few and compare. If you're out with a paper map and want to be sure, read the reference twice from different edge labels.

Finally, remember what the numbers are for. If you ever need help outdoors, a six-figure reference tells a rescue team where to look within a 100 m square. That is a far shorter search than "by the big tree near the top", and a far more useful sentence to say out loud.

Photo: Brigitte Pellerin / Pexels