Chain

To use the chain strategy, pick a box with only 2 candidates that you would like to solve. Now imagine that the answer to that box is the first candidate. Then add the effects of changing that box, and you will find yourself with a chain of numbers. Now think that the box is the other candidate. In most cases, a box or too in the chain will be the same number in both variations. You can now deduct that that square is solved.

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