The case notes
How a Zipper line works
Digits equally far from the line centre pair to the same total. On an odd line, that total is the centre digit. Watch a worked example, then try the idea in a puzzle.
Put it into practice
Pair cells around the centre
A Zipper line pairs cells the same distance from its centre, as if you folded the line in half.
Centre 9 = every pair sum
This line has five cells, so it has a single centre. The centre is nine, and each matching pair must add to nine.
8 + 1 = 9
The inner pair proves the pattern: eight and one make nine. Now move one step farther out on each side.
7 + 2 = 9
The outer pair must also make nine. Seven is already at one end, so the far end must be two.
Middle out. Equal pair sums.
Start in the middle, then work outward in mirrored pairs. On an odd-length Zipper, the centre digit is their total.
Read the transcript
A Zipper line pairs cells the same distance from its centre, as if you folded the line in half. This line has five cells, so it has a single centre. The centre is nine, and each matching pair must add to nine. The inner pair proves the pattern: eight and one make nine. Now move one step farther out on each side. The outer pair must also make nine. Seven is already at one end, so the far end must be two. Start in the middle, then work outward in mirrored pairs. On an odd-length Zipper, the centre digit is their total.
