The case notes
Let a modular line protect the box’s last one
This modular line crosses the top of a Sudoku box. We can use both rules together, before any digit on the line is solved. Watch a worked example, then try the idea in a puzzle.
Put it into practice
A modular line inside a box
This modular line crosses the top of a Sudoku box. We can use both rules together, before any digit on the line is solved.
Box 1 needs 1 on the line
The box needs a one. These two outside ones remove it from the box’s lower rows, so its one must be somewhere on the three-cell line.
Try 4 → the 147 slot is used
Try four at the start. Four belongs to the one-four-seven group. A modular line uses that group only once in these three cells, so the other two could not be one either.
Remove 4 from r1c1
Then there is no place for one in the box. So four cannot go at the start of this line. The line must leave its one-four-seven place available for one.
Read the transcript
This modular line crosses the top of a Sudoku box. We can use both rules together, before any digit on the line is solved. The box needs a one. These two outside ones remove it from the box’s lower rows, so its one must be somewhere on the three-cell line. Try four at the start. Four belongs to the one-four-seven group. A modular line uses that group only once in these three cells, so the other two could not be one either. Then there is no place for one in the box. So four cannot go at the start of this line. The line must leave its one-four-seven place available for one.
