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CONT: The most useful part of it is the top right corner, which has 1:50,000 scale map gradations (divisions of a grid square) marked along and down it, counting backwards from the corner. When you place the correct numbered marks on the grid lines of the map, the corner of the romer shows the point the map reference refers to. This is essential for accurate plotting of 6 and 8 figure map references. Most romers have a hole in them; it's a good idea to loop a piece of string through it and hang it round your neck so you don't lose it under the seat.