Coordinates

Before the book explains latitude and longitude, it shows you the same idea somewhere you already know it.

Imagine a big market with neat rows of shops, all the same size. You want to meet a friend at a stationery shop inside, but your friend does not know where it is. So you say: 'Meet me at 6 pm at the 7th shop in the 5th row from the entrance.' Now your friend can find you exactly.

Notice that you gave two numbers, and neither would be enough alone. 'The 7th shop' could be in any row; 'the 5th row' could be any shop along it. Together they pick out exactly one place.

Now a chessboard. To record the moves of good players, the columns are lettered a to h and the rows numbered 1 to 8. In the book's picture, white has just opened the game by moving the queen's pawn two squares forward, a very common opening. So the pawn has moved from d2 to d4. A letter and a number: again two, and again neither alone would do.

A system like this, where two values together fix one place, is called a system of coordinates. Thanks to their two coordinates, the stationery shop and the chess square can each be found exactly.

Maps use a similar system of coordinates to find any place on Earth. The Earth's two coordinates have longer names, latitude and longitude, but they work just like the row and the shop. A food-delivery app finds your house in the same way, from its coordinates.