Single Position

Look at the highlighted line.

Where can you place a 7?

In the left and middle box, there is both a 7 already, so we can't place another 7 here.

Because in the right box, there is not a 7 already, we have to check the columns here.

We see now that there is only one space left for a 7 (green square).

Exercise: Try to find the 8 in the the highlighted line!