
Hidden Singles by Box 

If a number only shows up as a candidate in one cell of a box, you may set that cell value to be that number. In the example below the candidate 6 is only found in cell I6 of the middleright box. Since each box must contain one 6, this cell must be the 6. This is a hidden single by box and is given a Sudoku Snake skill value of 1.
Like with hidden singles, this technique can be spotted without filling in the candidates by visualizing the elimination of units and seeing what remains. The above example is repeated below in this manner. By visualizing the elimination of the shown columns and row, it can be seen that, since all other empty cells in the middleright box are eliminated, cell I6 must be a 6.






