The case notes
How a modular line cycles
Any three consecutive line cells hold one digit from each group: {1,4,7}, {2,5,8}, and {3,6,9}. Watch a worked example, then try the idea in a puzzle.
Put it into practice
Three repeating groups
A modular line groups digits by what remains when you divide by three. Here is the simpler way to see it.
147 · 258 · 369
One, four, seven form the first group. Two, five, eight form the second. Three, six, nine form the third. Any three neighbouring line cells need one from each group.
The next cell joins 147
Four, eight, three have all three groups, even though they are not in number order. Slide along: eight and three leave the first group missing.
One or seven
That means one, four or seven. Four is already in this row, so the new cell is one or seven. The line has narrowed it to a pair.
Read the transcript
A modular line groups digits by what remains when you divide by three. Here is the simpler way to see it. One, four, seven form the first group. Two, five, eight form the second. Three, six, nine form the third. Any three neighbouring line cells need one from each group. Four, eight, three have all three groups, even though they are not in number order. Slide along: eight and three leave the first group missing. That means one, four or seven. Four is already in this row, so the new cell is one or seven. The line has narrowed it to a pair.
