The case notes
Reject a Renban digit that leaves its box empty
This Renban line sits along the bottom of a Sudoku box. The two ones outside remove one from its upper rows. The box must put one somewhere on the line. Watch a worked example, then try the idea in a puzzle.
Put it into practice
The box needs 1 on the line
This Renban line sits along the bottom of a Sudoku box. The two ones outside remove one from its upper rows. The box must put one somewhere on the line.
Try 9 → line {7, 8, 9}
Try nine in the first line cell. Three consecutive Sudoku digits that include nine can only be seven, eight and nine.
No place for 1 in the box
That set contains no one. But every other cell in this box has already lost one. The trial nine would leave the box without its one.
Remove 9 from r9c4
So nine cannot go in that line cell. A Renban can rule out a trial when its whole box needs a digit the line would exclude.
Read the transcript
This Renban line sits along the bottom of a Sudoku box. The two ones outside remove one from its upper rows. The box must put one somewhere on the line. Try nine in the first line cell. Three consecutive Sudoku digits that include nine can only be seven, eight and nine. That set contains no one. But every other cell in this box has already lost one. The trial nine would leave the box without its one. So nine cannot go in that line cell. A Renban can rule out a trial when its whole box needs a digit the line would exclude.
