Solving Classic Sudoku: 'Candidate Lines'

This technique is widely used to increase the speed of solving a sudoku. When a number has to be placed in one of the two (or more) cells of a box which are in a single row/column, the number cannot be placed in any other cell of that row/column.

Take a look at Puzzle No.75. Observe the '5' in the 1st box.


The possible positions of placing a '5' in the 2nd box is marked in green.


'5' has to be placed in either R1C4 or R2C4. Either way, '5' cannot be placed in any other cell of the 4th column, marked in red.


Using the '5' of the 6th column, there is only one unique position of placing a '5' in the 8th box, marked in green.

Other Solving Techniques