You can solve most soduku's with the methods described until now.

The toughest sudokus, however, require a supplementary strategy: the hypothesis. This goes for sudokus that contain multiple candidates for multiple cells, even after repeated application of the methods described.

In this case, make a copy of the current situation, in order to return to it when your hypothesis leads to contradiction.

After that, you look for a powerful candidate for a cell – mostly one an equal pair. Expand the sudoku after that, according to the methods of this site.

Sometimes you're lucky, and this doesn't lead to two conflicting candidates in a row, in a column or in a ninesome before the sudoku is completed.

However, when you encounter such conflicting candidates, you must return to the copy you made before and try the next candidate. Demo 2 provides an example of a sudoku ending in two paths...

In seldom cases, the hypothesis can run into trouble in a similar way and require a next hypothesis until the sudoku is solved.