A Singles Chain consists of an unbroken series of links, where a link is a line joining two candidates within a unit when there are
exactly two candidates of that value within the unit. In this chain, the candidate value is 2. For each link of the chain, one of the linked
candidates will be a solution, while the other will be a non-solution. All of the solution candidates have the same color (either green or
red) while all of the non-solution candidates have the other color.
The cell highlighted in blue at G2 is in the same column as a green 2 and in the same row as a red 2. Now either the green 2 will be a
solution or the red 2 will be a solution. We have no way of knowing at this stage which way it will be, but we can be very sure that the
candidate 2 at cell G2 cannot be a solution. It has a background of magenta, indicating that it may be removed.
Rule 1 states that a candidate which can "see"
two like numbered candidates of differing colors cannot be a solution.