All Sudoku Variants
9×9 · Constraint

Outside Sudoku

Also known as Outside Clue Sudoku

Clues outside the grid list digits that must appear in the first three cells of that row or column, nearest the clue.

5 3 4 1 9 8 4 33 4 51 8 9

An example Outside puzzle: the clue 3 4 5 (row 1) means a 3, 4 and 5 fill the first three cells in some order.

Outside Sudoku writes small clue digits outside the grid, beside some rows and columns. Those digits must all appear somewhere in the first three cells of that row or column — the nearest 3×3 box — in any order. Unlike sum-based edge clues, an Outside clue tells you exactly which digits are near the border, which is often even more useful.

The rule: the clue digits sit in the nearest three cells

Every digit written outside a row or column must show up among the three cells closest to it. Below, the clue "3 1" means both a 3 and a 1 appear somewhere in the first three cells — here as the 3 and 1 in positions two and three.

3 1 5 3 1

The clue "3 1" means a 3 and a 1 appear in the first three cells (here in cells two and three).

A worked example, step by step

The strongest case is when the clue lists three digits — then the first three cells are pinned exactly. Take the clue "1 2 3".

1 2 3

Step 0. The clue lists three digits: 1, 2 and 3.

Step 1 — locate the three cells. All the clue digits must live in the first three cells, so those are the cells in play.

1 2 3

Step 1. The three clue digits must all fit in the first three cells (highlighted).

Step 2 — the box is full. Three digits into three cells leaves no room for anything else: the first three cells are exactly 1, 2 and 3, in some order set by the rest of the grid.

1 2 3123123123

Step 2. The first three cells are exactly {1, 2, 3} — the rest of the box is decided elsewhere.

Reading fewer clue digits

Shorter clues are looser but still helpful. A clue with two digits — say "9 8" — tells you both a 9 and an 8 sit in the first three cells, so the third cell is something else and, crucially, a 9 and an 8 are kept out of the rest of the row. A single-digit clue simply guarantees that digit appears in the near box, which often combines with the ordinary rules to fix its exact cell.

Solving strategy

  • Prize the three-digit clues. They fill the first three cells with an exact set, just like a solved box triple.
  • Use clues to exclude. A digit forced into the near three cells cannot appear anywhere else in that row or column.
  • Cross-reference. A big clue digit (9 or 8) pinned near one edge helps the crossing columns place their own copies.
  • Feed the grid. Each digit an Outside clue fixes behaves like a given for its row, column and box.

Common mistakes

  • Fixing the order. The clue says which digits are near the edge, not the sequence — their order is decided by other constraints.
  • Looking beyond three cells. Outside clues only ever concern the nearest three cells (the border box).
  • Forgetting the exclusion. A digit pulled into the near three cells is thereby banned from the rest of that line.
  • Ignoring ordinary Sudoku. The near cells still can't repeat a digit in their row, column or box.

Can I play it here?

Not yet — Outside 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: