Empty Rectangles

Index
Empty Rectangles
The Empty Rectangles technique can be used to eliminate a candidate value. An important part of this pattern is a box with a candidate value that only occurs in two lines. In the example below only the candidate values 4 are visible. The yellow marked cells in the box on the top right are the only cells containing the value 4. These can be marked by only two lines, marked red in the example.
Empty Rectangles
The second part we need is a row or column containing the candidate value twice. In the example below a column with the two cells is marked red.
Empty Rectangles
The last part we need is a cell to complete the rectangle. In the example below a line is marked red that leads to the finishing cell. The Empty Rectangle is now complete.
Empty Rectangles
In the image below is shown what can be achieved with this pattern. When the cell on the bottom right is a 4, the cell on the bottom left cannot be a 4. When the cell on the bottom left is not a 4, the cell on the top left must be a 4. When the cells on the bottom right and the top left both are 4 then there are no candidate values 4 left in the top right box. The bottom right cell cannot be a 4, this candidate value can be excluded.
Empty Rectangles