Solving Classic Sudoku: 'Two Pairs'

This is a common technique used by top sudoku solvers. If you find a number occuring in the same to cells of two different adjacent boxes, then that number cannot occur in any other cell of the corresponding row/column of those two cells.

Take a look at Puzzle No.2. Try solving the centre box.


Using the earlier strategies, the centre box can be easily completed.


Observe the highlighted '4's and marked red cells which indicate cells where '4' cannot occur. Hence in the 2nd and 3rd boxes, the only possible cells where '4' can occur is marked in green.


Since '4' has to be placed in those two pairs of cells which are identical in adjacent boxes, '4' cannot be placed in any of the remaining cells of those two rows marked in red.


Observe the 1st box and using the '4' of the 1st column, there is a unique cell left where '4' can be placed.

Other Solving Techniques