Question 1

What is a Naked Single in Sudoku?

Accepted Answer

A Naked Single is when only one number can possibly go in a cell. Every other digit from 1–9 is already used somewhere in that cell's row, column, or 3×3 box. The remaining digit must go there — there's no other choice.

Question 2

What is a Hidden Single in Sudoku?

Accepted Answer

A Hidden Single is when a digit can only go in one cell within a row, column, or 3×3 box. Even if that cell has other candidates, this digit is blocked from every other empty cell in that unit — so it must go here.

Question 3

What is an X-Wing in Sudoku?

Accepted Answer

An X-Wing occurs when a digit appears as a candidate in exactly two cells in each of two rows, and those cells share the same two columns. The digit must go in one of the four corner cells, which means it can be eliminated from all other cells in those two columns.

Question 4

What is a Swordfish in Sudoku?

Accepted Answer

A Swordfish is like an X-Wing extended to three rows and three columns. If a digit appears in only 2 or 3 cells across three rows, and all those cells fall within the same three columns, the digit can be eliminated from all other cells in those three columns.

Question 5

What is an XY-Wing in Sudoku?

Accepted Answer

An XY-Wing uses three cells. A pivot cell has two candidates [A,B]. Two wing cells each share one of those digits plus a common digit C. No matter which value the pivot takes, one wing will always be C — so any cell that sees both wings cannot be C.

Question 6

What is a Unique Rectangle in Sudoku?

Accepted Answer

A Unique Rectangle (also called a Gordonian Rectangle) uses the rule that a valid Sudoku must have exactly one solution. If four cells form a rectangle across two boxes, and three of them have only two identical candidates, the fourth cell must use a different digit — otherwise the puzzle would have two solutions, which is impossible.

Question 7

What are Forcing Chains in Sudoku?

Accepted Answer

Forcing Chains test assumptions. Pick a cell with two candidates A and B. Follow the logical consequences of each choice. If both chains lead to the same conclusion elsewhere, that conclusion must be true. If one chain leads to a contradiction, the other candidate is forced.

Question 8

What is Nishio in Sudoku?

Accepted Answer

Nishio is a specific form of trial-and-error: assume a candidate is placed in a cell and propagate all consequences. If this leads to a cell with no valid candidates — a contradiction — then that assumption was wrong and the candidate can be eliminated.

Question 9

How does this Sudoku solver work?

Accepted Answer

Enter your puzzle by tapping cells and using the number pad, or paste an 81-digit string. The solver first explains how candidates are calculated, then works through the puzzle one step at a time. Each step highlights the relevant cells in colour and explains the logic in plain English — gold for the answer cell, blue for the scanned row/column/box, pink for blocking cells.

Question 10

Can this solver handle any difficulty Sudoku?

Accepted Answer

Yes. The solver uses 16 techniques in order of difficulty: Naked Single, Hidden Single, Naked Pair, Hidden Pair, Naked Triple, Pointing Pair, X-Wing, Swordfish, XY-Wing, XYZ-Wing, Unique Rectangle (Types 1 and 2), BUG+1, W-Wing, Nishio, Forcing Chains, and Bifurcation as a last resort. This covers every published Sudoku puzzle from beginner to diabolical.

Question 11

What do the colours on the grid mean?

Accepted Answer

Gold with a ring = the answer cell being solved. Blue wash = the row, column, or box being scanned. Pink/rose = filled cells whose digits are blocking other options. Purple ring = paired or grouped cells in techniques like Naked Pairs or XY-Wing.

Sudoku Solver — Step by Step, Every Technique Explained

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Tier 1 — Basic techniques

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