The case notes
Rule out an impossible modular group
A modular line repeats three digit groups: one-four-seven, two-five-eight and three-six-nine. On this long line, a group appears three times. Watch a worked example, then try the idea in a puzzle.
Put it into practice
147 · 258 · 369
A modular line repeats three digit groups: one-four-seven, two-five-eight and three-six-nine. On this long line, a group appears three times.
Three places · three different digits
The first, fourth and seventh line cells share a group. They also share one Sudoku row, so those three cells must hold different digits.
Only 4 and 7 for 3 cells?
Could their group be one, four, seven? One is already in this row, outside the line. That leaves only four and seven for three cells. Two digits cannot fill three different places.
Not the 147 group
So the one-four-seven group cannot go on those three positions. They must use one of the other groups. Four and seven are ruled out of all three cells.
Shared group + row sight
The line gives the shared group. The row tests whether that group has enough different digits to fill its three places.
Read the transcript
A modular line repeats three digit groups: one-four-seven, two-five-eight and three-six-nine. On this long line, a group appears three times. The first, fourth and seventh line cells share a group. They also share one Sudoku row, so those three cells must hold different digits. Could their group be one, four, seven? One is already in this row, outside the line. That leaves only four and seven for three cells. Two digits cannot fill three different places. So the one-four-seven group cannot go on those three positions. They must use one of the other groups. Four and seven are ruled out of all three cells. The line gives the shared group. The row tests whether that group has enough different digits to fill its three places.
