Sudoku by Big T

Every row, column, and 3×3 box holds the digits 1–9 exactly once.

Nothing is ever a guess — every puzzle here can be solved by logic alone.

Tap a cell, then tap a number — or use Fill mode to work the other way round.

Pick a number, then tap cells to drop it repeatedly.

Also a scanning trick: lock a digit and sweep the board to spot everywhere it can — and can't — go.

Jot candidate numbers in a cell before committing.

Handy for narrowing a cell to two or three options before you choose.

Fix a slip, step back, or restart the puzzle.

Names the next logical move — without giving away the answer.

Free and unlimited. It highlights the cells to look at and explains the technique that cracks them; you still place the digit yourself.

A cell where only one number can possibly fit.

Its row, column, and box already use the other eight digits, so a single number is left for that cell. Fill it in.

A number that fits in only one cell of a row, column, or box.

Don't look at the cell's own candidates — scan the whole unit. If a digit can go nowhere else in that row, column, or box, it belongs in that cell, even when the cell still shows other candidates.

Two cells that lock two numbers between them.

When two cells in a unit share the same two candidates, those digits are theirs alone — clear them from the rest of the unit. Triples work the same way with three cells and three digits.

A number trapped on one line inside a box.

If a digit can only sit on a single row or column within a box, it can't appear elsewhere on that line — rule it out there to open things up.

X-Wing, Swordfish, and XY-Wing turn up in Hard and Expert.

These use shapes spanning several rows and columns to eliminate a digit. The Hint button will walk you through them when one is the next logical step.

Pick a digit and sweep every box for the places it cannot go.

Take the 1s: any row or column already holding a 1 rules that digit out of every cell it passes through. Cross those off across all nine boxes and the boxes where only one cell survives hand you a placement. Working one digit at a time across the whole grid finds more than studying one cell hard does, and it is the fastest way through Easy and Medium.

If a digit can only sit on one line within a box, it cannot sit elsewhere on that line.

Suppose the 4s in the top-left box are confined to its middle row. You do not know which cell is the 4, but you know the 4 for that box is somewhere in that row — so no other box can put a 4 in the rest of that row. This eliminates rather than places, and eliminations are what unlock the next single. The reverse also works: if a digit in a row is confined to the cells inside one box, it can be cleared from the rest of that box.

Two cells sharing exactly two candidates own those digits between them.

If two cells in a row can only be 3 or 7, then 3 and 7 live in those two cells in some order, and both can be cleared from every other cell in that row. That is a naked pair. A hidden pair is the same fact wearing a disguise: two digits that can only go in two cells of a unit, even though those cells still show other candidates — the extra candidates can be cleared. Triples work identically with three cells and three digits, and do not require every cell to show all three.

Four cells at the corners of a rectangle eliminate a digit from two whole lines.

Find a digit with exactly two possible positions in each of two different rows, and those positions in the same pair of columns. Whichever way the digit falls, it occupies one corner in each row — so it cannot appear anywhere else in those two columns. Columns and rows are interchangeable here. This is the first technique that reads as a shape rather than a scan, and it is where Expert begins.

An X-Wing stretched across three rows and three columns.

Same logic, one size up: a digit confined to the same three columns across three rows can be cleared from the rest of those columns. The candidate positions do not all have to be present — two of the three columns in a given row is enough.

Three two-candidate cells that force a digit out of the cells they both see.

A pivot cell holding XY sees one cell holding XZ and another holding YZ. Whichever way the pivot resolves, one of the other two becomes Z — so any cell seeing both of them cannot be Z. It is the one common technique that does not depend on rows and columns lining up, which is what makes it easy to miss.

Every puzzle here is solvable by logic, so a guess is never required.

If nothing is moving, the usual cause is a missed elimination rather than a need for a harder technique. Re-scan the digit you have placed least often, then check each box for a digit confined to one line. The 💡 button will name the easiest technique that fires next if you would rather be pointed at it — it names the move without making it.

Each puzzle is solved by a logic engine as it is generated, and graded by the hardest technique that solve needed.

Counting givens is the usual shortcut and it is a poor one — a 30-given puzzle can be easier than a 34-given one. Grading by the techniques actually required means a difficulty label describes the thinking, not the visual density.

Both need only singles. They differ in how much scanning you do.

Neither requires a technique beyond naked and hidden singles. Easy keeps at least 44 givens so the next placement is usually close by; Medium drops to 36, which means longer scans and more of them, but never a new idea to learn. That gentle step is deliberate.

The first real jump: locked candidates and triples.

Singles alone stop being enough. Hard puzzles need pointing pairs or a naked triple somewhere in the solve, and sit at 26 givens or more.

X-Wing, Swordfish and XY-Wing territory, with no density floor.

Expert puzzles need at least one of the shape-based techniques and are left as sparse as the logic allows. They are still solvable without guessing — that is a guarantee of how they are generated, not an aspiration.

Flags cells that conflict with the solution.

Show or hide the solve timer.

Greys a digit's button once all nine are placed.

Switch between light and dark, and pick a color theme.

The occasional passive nudge when you've been stuck a while.

Tips appear on their own, fade away by themselves, and stop showing up once you've seen them a couple of times. Turn them off entirely in Settings.

Landscape only. Puts the number pad under your dominant hand.

Portrait only. Brings the board into easier thumb reach.