The case notes
Count a modular class across a long line
On a modular line, each run of three cells uses one digit from each group. A long line lets that pattern repeat. Watch a worked example, then try the idea in a puzzle.
Put it into practice
147 · 258 · 369
On a modular line, each run of three cells uses one digit from each group. A long line lets that pattern repeat.
Positions 1, 4, 7 share a group
The first, fourth and seventh line cells use the same group. Watch the three-cell window slide: each new third cell replaces one from that group.
This group is {1, 4, 7}
The first of those cells is one. Its group is one, four, seven. The other two highlighted cells must also come from that group.
The row’s 147 group is used
They share one Sudoku row, so they cannot repeat. These three places consume one, four and seven, in some order. No more digits from that group remain for this row.
Outside: no 4 or 7
The last two cells sit outside the line. Neither can be four or seven; both are reserved by the line. We have removed candidates without placing those line digits.
Read the transcript
On a modular line, each run of three cells uses one digit from each group. A long line lets that pattern repeat. The first, fourth and seventh line cells use the same group. Watch the three-cell window slide: each new third cell replaces one from that group. The first of those cells is one. Its group is one, four, seven. The other two highlighted cells must also come from that group. They share one Sudoku row, so they cannot repeat. These three places consume one, four and seven, in some order. No more digits from that group remain for this row. The last two cells sit outside the line. Neither can be four or seven; both are reserved by the line. We have removed candidates without placing those line digits.
