Sudoku rules, techniques and how to finish a grid

What is Sudoku?

Sudoku is a 9×9 number puzzle. DIGIGMA uses the classic form: nine rows, nine columns, and nine 3×3 boxes. Some digits are already given. Those cells never change.

A cell is one square. A box is a 3×3 block of nine cells. Keep those words apart: the box is the group, the cell is the single square.

Goal of the game

Fill every empty cell with a digit from 1 to 9 so the finished grid obeys every rule. DIGIGMA Sudoku has exactly one solution. You do not need to guess.

Rules

• Use digits 1 through 9 only.
• Each row contains 1 through 9 once.
• Each column contains 1 through 9 once.
• Each 3×3 box contains 1 through 9 once.

A digit is legal in a cell only if it is still free in that cell’s row, column, and box.

How to start a grid

Open an Easy board from available grids. Tap a cell, then pick a digit. If you are not sure, turn on Draft and mark the candidates that still fit.

Start with the most constrained units: a row, column, or box that is almost full. Forced digits appear there first.

A step-by-step solving method

1. Scan rows, columns, and boxes that are almost complete.
2. Find cells that accept only one digit (naked singles).
3. Use Draft to note leftover candidates in harder cells.
4. Eliminate candidates that a new placement has made impossible.
5. Look for a digit that has only one remaining place in a row, column, or box (hidden singles).
6. Repeat. Each placement should shrink the next set of options.

Stay on this loop. The next forced cell is usually close to the last one you wrote.

Core solving techniques

Beginner — Naked single

A cell has only one candidate left. Write it. In this 3×3 box eight digits are already present, so the empty cell must be 4.

Beginner — Hidden single

A digit still has several empty cells in theory, but only one of them can actually hold it inside this box, row, or column. Write it there even if the cell still looks open for other digits.

Beginner — Candidate elimination

When you place a 6, remove 6 from the Draft of every cell that shares its row, column, or box. If a cell drops to one mark, it has become a naked single.

Intermediate — Naked pair

Two cells in the same unit share exactly the same two candidates, and nothing else. Those two digits are locked there. Remove them from every other cell in that row, column, or box.

Intermediate — Hidden pair

Two digits appear as candidates in only two cells of a unit. Those cells can hold only those two digits. Clear any extra marks from them.

Intermediate — Box-line interaction

If a digit’s candidates in a box all sit on one row, that digit cannot appear elsewhere on the same row outside the box. The same idea works with a column. This is also called a pointing pair.

Harder patterns exist (X-wing, swordfish). You will not need them on Easy or Medium DIGIGMA boards. If you stall, return to singles and clean Drafts before chasing rare techniques.

When you’re stuck

• Rescan every almost-complete row, column, and box.
• Re-check Draft marks and delete any digit that is now impossible.
• Hunt hidden singles: pick one digit and walk each box.
• Look for a box-line interaction on a digit that feels crowded.
• Do not guess. A DIGIGMA grid always has a forced next step.

Hint shows a next idea if you want help. Reveal fills the grid. Neither is required to solve.

Common mistakes

• Guessing too early instead of scanning for a single.
• Watching the row and forgetting the 3×3 box.
• Keeping a Draft candidate after it has been placed nearby.
• Filling every cell with marks and never cleaning them.

Practical tips

• Each correct digit creates the next one. Stay local.
• When you stall, pick one digit and hunt it through the nine boxes.
• Draft is a memory aid, not a score. Fewer clean marks beat a crowded cell.
• If you want a quieter session, Read has a short note on why guessing feels worse than scanning.

FAQ

Is guessing necessary in Sudoku?

Not on a DIGIGMA grid. Every board has one solution and a path of forced placements.

What should I do when I get stuck?

Clean Drafts, hunt singles, then look at one digit across the boxes. Guessing is the last thing to try — and you should not need it.

What are pencil marks?

On DIGIGMA they are called Draft. They are candidate digits you write in a cell until only one remains.

Can a Sudoku have more than one solution?

DIGIGMA puzzles do not. One finished grid, one path of logic.

How long should a Sudoku take?

As long as it takes to stay calm. Time is private. Easy boards are a few quiet minutes; harder bands take longer.