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Solve Sudoku Using Crosshatching

Scanning or Crosshatching is a method that is very well fit to solve a Sudoku by hand.

First you chose a number you want to find a place for, let's say a One

Now you chose a 3x3-sub grid in which no One is present. As an example, we chose the second sub grid from above on the left-hand side.

A Sudoku with a selected 3x3-sub grid, marked in bright green.

In the left three columns and in the middle three rows we search for Ones and mark them by vertical/horizontal lines:

We search for Ones in each row/col that touches the selected 3x3-sub grid and mark the corresponding row/col.

In any marked cell there must not be entered a One. In our selected 3x3-Subgrid only one cell remains. There we enter a One.

In the selected 3x3-sub grid remains only one cell where we may enter a One.

We have just found a place for one number. Further Scanning for Ones doesn't reveal another position, but scanning for a Two helps...

A common algorithm is to scan for each number consecutively.