The case notes
How marked diagonals work
Digits cannot repeat on the marked diagonal. Watch a worked example, then try the idea in a puzzle.
Put it into practice
One extra group
A marked diagonal is one more place where digits cannot repeat. Follow this line from the top-left corner.
Only one 5 on the line
The first diagonal cell already holds five. Every other cell on this marked line must use a different digit.
The centre cannot be 5
Look at the centre. It is far from that five and shares no row, column or box. The diagonal still rules out five.
Check the marked lines
Some puzzles mark both long diagonals; some mark one. Apply the no-repeat rule to each diagonal the puzzle marks.
Read the transcript
A marked diagonal is one more place where digits cannot repeat. Follow this line from the top-left corner. The first diagonal cell already holds five. Every other cell on this marked line must use a different digit. Look at the centre. It is far from that five and shares no row, column or box. The diagonal still rules out five. Some puzzles mark both long diagonals; some mark one. Apply the no-repeat rule to each diagonal the puzzle marks.
