All Sudoku Variants
9×9 · Constraint

Lockout Lines Sudoku

Also known as Lockout Sudoku

The two diamond endpoints of a line must be far apart, and no digit on the line may fall between them.

5 32918

An example Lockout Lines puzzle: the two endpoints differ by at least 4 (2 and 8); cells between avoid both and their whole range — here 9 and 1.

Lockout Lines Sudoku marks each line with two diamond endpoints. Two rules bind them: the diamonds must differ by at least 4, and no digit on the line between them may equal either diamond or fall in value between the two. The line is "locked out" of the whole range its endpoints span.

The rule: stay outside the diamonds' range

Read the two diamond values; every cell on the line between them must sit strictly below the smaller diamond or strictly above the larger. Below, the diamonds are 2 and 8, so the line cells avoid everything from 2 to 8 — leaving only 1 and 9.

2 1 9 8

Diamonds 2 and 8 (red) differ by 6; the line cells avoid 2–8, so they are 1 and 9.

A worked example, step by step

Take a line whose two diamonds are already solved as 2 and 8.

2 8

Step 0. The diamonds are 2 and 8; the two line cells are empty.

Step 1 — find the locked-out range. The diamonds span 2 to 8, so the line cells cannot be any of 2, 3, 4, 5, 6, 7 or 8. That rules out seven of the nine digits in one stroke.

Step 2 — only the extremes survive. The only digits left are 1 (below 2) and 9 (above 8), so both line cells must come from {1, 9}.

21919 8

Step 2. Only 1 and 9 lie outside the 2–8 range — the line cells are {1, 9}.

How far apart the diamonds are

The wider the diamonds, the more the line is squeezed. Diamonds of 1 and 9 lock out everything between, leaving no legal digit at all — so that combination is impossible and the diamonds can never be 1 and 9 together. Diamonds exactly 4 apart, like 3 and 7, leave the most room (1, 2, 8, 9). Reading the gap first tells you how tight the line will be.

Solving strategy

  • Solve the diamonds first. Two known endpoints instantly fix the forbidden range for the whole line.
  • Delete the middle band. Strike every digit from the smaller diamond up to the larger out of the line cells.
  • Watch the 4-gap minimum. The diamonds themselves must differ by at least 4 — use it to rule pairs in or out.
  • Feed the grid. Each digit fixed on the line behaves like a given for its row, column and box.

Common mistakes

  • Including the diamond values. The line avoids the diamonds and everything between them — the endpoints are excluded too.
  • Forgetting the 4-gap. The two diamonds must themselves be at least 4 apart.
  • Diamonds of 1 and 9. That would leave the line no legal digit — it can't happen.
  • Forgetting ordinary Sudoku. Line cells still can't repeat within a row, column or box.

Can I play it here?

Not yet — Lockout 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: