Charts for the west coast will have an easterly variation noted in the middle of the compass rose. Charts for the east coast will have a westerly variation. They aren't interchangeable in the way I think you mean. If you're using a chart for say, the entrance to New York Harbor, you'll use the westerly variation as noted in the compass rose.
They give you both east and west so that the publishers can use the same example on either coast. But you need to use the one that's applicable to your area.
You'll have to account for the change in variation over the years regardless of which direction you are going (magnetic to true or true to magnetic).
I hope this helps.
A great book that I recommend is How to Read a Nautical Chart by Nigel Calder. Good explanations and it also contains all of Chart No. 1.
Thanks, I'll check that book out. I just assumed that if I buy charts for any given area, the variance would be noted on it whether it was E or W. Most of my navigating has been piloting by landmarks or by GPS. I'm trying to self-teach myself on the subject. Using electronics isn't all that difficult but I want to learn how to do it without having to totally rely on them. Would you recommend Calder's book as a good source to learn not just how to read a chart but how to figure course, set, drift, accounting for currents, tides, etc. and all those other things that I should know but don't yet know that I should know?