As outlined in the “Back To Basics" section Magnetic North is not located in the same position as True North. The lines on this diagram indicate the lines of equal magnetic differences over the Earth. (For Quiz Night enthusiasts they are called Isoganals )

For example most of Europe is around 5”W  meaning the Variation is 5 degrees West, so the variation ( Angular difference between Magnetic and True North  ) is quite small - hence, many users ignore it completely.
However look at the difference between the East and West coasts of the USA ! >>
To  very accurately set up your compass to compensate for Variation on the compass base there is a small scale either side of the North indicator >>
Having determined what the variation error is in your location, line up the compass N arrows like this Then, taking for an exaggerated example, Vancouver Canada which has a variation of 20 degrees E (East ), rotate the compass base until the compass arrow lines up with Var. E 20
The sunrise/set position will now be accurately indicated by the Month lines radiating from the centre

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These direction lines are accurate to within 3 degrees for all locations within the band of 45-55 degrees of latitude >>
 
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