All Sudoku Variants
9×9 · Constraint

Quadruple Sudoku

Also known as Quad Clue Sudoku, Quadruples

A small clue printed where four cells meet lists digits that must appear among those four cells.

7 1 4 2 5 91 4

An example Quadruple puzzle: every digit in a circle must appear in at least one of its four touching cells — here 1 and 4.

Quadruple Sudoku prints small clues in circles that sit where four cells meet. Each digit inside a circle must appear in at least one of the four surrounding cells. A circle can list one digit or several — and when it lists four, it pins that little 2×2 corner completely.

The rule: the circle's digits live in the four cells

Read the digits in a circle; every one of them has to show up somewhere among the four cells touching that circle. Below, the circle holds 1 and 2, and indeed a 1 and a 2 sit in two of its four cells.

1 2◯12

The circle "1 2" means a 1 and a 2 appear among its four cells.

A worked example, step by step

The clue is strongest when it lists four digits. Take a circle holding 1, 2, 3 and 4.

◯1234

Step 0. The circle lists four digits: 1, 2, 3 and 4.

Step 1 — four digits need four homes. Each of 1, 2, 3 and 4 must appear in the four surrounding cells. But there are exactly four cells and four digits — so there is no room for anything else.

Step 2 — the corner is fixed. The four cells must be precisely 1, 2, 3 and 4, one each, in some order settled by the rows, columns and boxes that cross them.

1234123412341234◯1234

Step 2. The four cells are exactly {1, 2, 3, 4} — the whole corner is locked.

Reading shorter clues

A circle with fewer digits is looser but still useful. A single-digit circle guarantees that digit somewhere in the four cells — often enough, with the ordinary rules, to place it exactly. A repeated digit in a circle (say "2 2") is a stronger statement still: the digit must appear twice among the four cells, which forces it into two of them.

Solving strategy

  • Prize the four-digit circles. They fill their 2×2 corner with an exact set of four.
  • Combine with the crossing units. A digit a circle demands, narrowed by the rows and columns through those cells, often lands in one place.
  • Watch repeats. A digit listed twice must occupy two of the four cells — a powerful extra constraint.
  • Feed the grid. Each digit a circle helps place behaves like a given for its row, column and box.

Common mistakes

  • Reading "appears" as "in order". The circle says which digits are present in the four cells, not where each one goes.
  • Missing a repeat. A digit written twice must appear twice among the four cells — don't treat it as once.
  • Applying it beyond four cells. A circle only ever concerns the four cells that touch it.
  • Forgetting ordinary Sudoku. Those four cells still obey their rows, columns and boxes.

Can I play it here?

Not yet — Quadruple 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: