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.
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.
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.
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:
- Browse all the Sudoku variants we do offer.
- Or print free puzzles from the printable PDF page.
Where to play it
Quadruple Sudoku belongs to the constraint family and runs on 9×9. It is not one SudokuStreak generates yet - but the deduction it trains is the deduction every Sudoku runs on.
Difficulty and type are two separate dials here: the type sets the extra rule, the level sets how much of the grid you start with. Expert Sudoku leaves 25 of the 81 cells filled, where the beginner tools dry up almost entirely; Evil Sudoku leaves 23, with no easy moves at all. Take it at the gentler end while the pattern is still new, and move up once it stops being a fight.