An example Zipper Lines puzzle: on the violet line, digits equidistant from the centre (violet) add to it — here 1 + 5 = 6 and 2 + 4 = 6.
Zipper Lines Sudoku draws a line with a marked middle cell. Imagine folding the line in half at that centre: each pair of cells that meet must add up to the digit in the centre cell. Like a zip pulling two sides together, every equidistant pair shares the same total — the centre value.
The rule: pairs fold to the centre
Take the cell in the middle of the line. Then the two cells one step out from it must sum to that middle value; so must the two cells two steps out, and so on. Below, the centre is 9, and each outer pair — 4 + 5 and 2 + 7 — adds to 9.
Centre 9; each fold pair adds to it: 4 + 5 = 9 and 2 + 7 = 9.
A worked example, step by step
Take a five-cell zipper whose centre cell is already a 9.
Step 0. The centre cell is 9; the four outer cells are empty.
Step 1 — each fold pair sums to 9. The two cells either side of the centre add to 9, and so do the two outermost cells. Both pairs share the same target.
Step 1. The inner pair sums to 9, and so does the outer pair.
Step 2 — list the pairs. Two different digits adding to 9 are {1,8}, {2,7}, {3,6} or {4,5} — and never a 9, since that is the centre. Each fold takes one of those pairs, so the four outer cells are heavily constrained before any other clue.
Step 2. Every outer cell is 1–8 (never 9); each fold pair is one of {1,8},{2,7},{3,6},{4,5}.
Reading the centre
The centre digit is always the largest value the line can carry, because it equals the sum of two other positive digits — so a zipper centre is never small. A centre of 9 means every fold is a pair from {1,8} up to {4,5}; a centre of 5 restricts folds to {1,4} or {2,3}, pushing small digits onto the line. Reading the centre first tells you the whole line's flavour.
Solving strategy
- Find the centre value. It caps the whole line and sets every fold's target sum.
- Pair from the outside in. Match equidistant cells and treat each as a small two-cell sum.
- Exclude the centre digit. No outer cell can equal the centre, since two positive digits can't sum to it and reuse it.
- Feed the grid. Each digit a fold fixes behaves like a given for its row, column and box.
Common mistakes
- Pairing the wrong cells. Fold pairs are equidistant from the centre, not simply neighbours.
- Letting an outer cell equal the centre. The centre is the pair total, so it can't reappear as one of the paired digits.
- Forgetting the centre is fixed. One centre value governs every fold on that line.
- Ignoring ordinary Sudoku. Line cells still can't repeat within a row, column or box.
Can I play it here?
Not yet — Zipper Lines Sudoku isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile:
- Browse all the Sudoku variants we do offer.
- Or print free puzzles from the printable PDF page.