Sudoku Tutorial
X Chain

An X chain follows a single candidate (2 in
this example).   The chain is a sequence of links
between cells where:
  1. The chain must begin and end with strong links
  2. There must be an odd number of links in the chain
  3. All of the odd numbered links must be strong (shown in blue)
This means that one of the ends of the chain must
be a 2.  This will eliminate the slashed 2's.  (It is possible that both ends of the chain are 2's)

In this example, all of the links are strong links except in column 1. 

Here is another examples of an X chains

X Chain #2
X Chain #3

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