This is an extension of the
X-wing idea. In row 1, 1 must be the solution for one of the highlighted
cells. Exactly the same can be said in connection with row 6 and row 9.
If you now rotate your thinking through 90 degrees, you will conclude that in all of columns 5, 6 7, the 1 must
appear as the solution in one of the highlighted cells. Any 1s in columns 5, 6 and 7, other than those in the
highlighted cells cannot be part of the solution and may be removed. There are a total of seven instances in which
a 1 can be removed, and we are lucky that in one of these cases (C7-R3) we are left with a
Single Candidate
situation.