On this map there are lots of lines going up and down and side to side, these are called grid lines. Each of these grid lines has a number so you can tell which line you are talking about. 

If you want to talk about a particular square on the map you can use these grid lines to help you. 

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For example if you look at the map of ports below - Port G is in the found in a square.

The way you would describe the square is 18,69

 Notice how I wrote down the side to side grid line number first, then the up and down grid line number.