This pattern can help solve harder puzzles by finding twins and ghost numbers
Finding and using the Plus pattern will help you find twins and ghost numbers. These twins and ghost numbers will help you find more numbers in the rows and columns where the Plus pattern exists.
The Plus pattern exists when a box has numbers in all four corners. Look at boxes 2 and 8 in figure 1 below. These two boxes each have four numbers in them, and every number is in a corner. Box 2 has 6148 and box 8 has 5639. The empty cells form a 'Plus.'
Keep in mind that a box can still have a Plus pattern even if some of the non-corner cells are populated, as long as all of the corner cells are populated.
This puzzle is an easy one but it illustrates the Plus pattern well. Note that the 7 in box 7 shows that the only place a 7 can go in box 8 is in row 7 and 9. In fact the 7 goes in row 9 because there is already a 7 in row 7.
Figure 2 below shows the 7 in row 9 (7 circled red).
Figure 2 also shows how the 7 in row 9 helps us in box 2. Because the 7 in row 9 is in column 5 the 7 in box 2 must be in row 2. The red 7s show the only two possible cells for a 7 in box 2 (these are called ghost 7s, see the ghost numbers technique if you aren't familiar with it). And because we know this we know that the 7 in box 3 is in row 1 (7 circled red).
If you look closer at box 2 you will notice that the two cells in row 2 are actually 57 twins (see the twins technique if you're not familiar with twins). This is because there is a 5 and a 7 in column 5 outside of box 2. And the 57 twins can be solved. See figure 3 below (the 5 and 7 in box 2 are circled red).
You should now understand how the Plus pattern can help you find both twins and ghost numbers. See the video below for more Plus pattern examples.
Watch this video for several good examples of the Plus pattern
See the video index to see more videos like this one.
Sudoku - the puzzle that addicts