Introduction Cross-Hatching Counting 1-9 Cross-Hatching 2 Pencil Marks Row and Column Limits Block Interaction 'N'-Possible Hidden Subset X-Wing Swordfish Cell Colouring Forcing Chains Trial and Error

Cross-Hatching 2

The technique we now discuss is a variant of the Cross-hatch / Hidden Single method discussed previously, but it is rarely documented - probably because it is a bit harder for a human to 'see' this in a Sudoku grid.

Just as the cross hatching technique finds that there is only one possible placement for a particular value in a subgrid, this technique may result in finding that there is only one possible location for a value in a row or column.

For example, concentrate on possible values that can go in the green cell marked Y:

At first glance, it seems that possible values on this cell are {4,5,6,8} (using the counting technique). However, if we specifically look for possible locations for value 8 on the column, we see that:

• Neither of the cells in the top-center subgrid can be 8 because there is an 8 in that subgrid already;
• The free cell in the central subgrid can not be 8 because there is an 8 on the row.

Therefore, the cell marked with the Y is the only possible placement for value 8 on this column... so we can fill it in.

So in summary, this is logically almost exactly the same as the cross-hatch method, but in practice you are less likely to use this technique as it seems to be harder for us humans to see the significant arrangements of digits in a row or column, as opposed to intersecting lines in a subgrid.