Sudoku Techniques

Sudokuno teaches techniques inside complete puzzles rather than through isolated diagrams. You get a real Sudoku, and the hints reveal the structure in three steps before they will make the move for you.

Every puzzle is graded by the technique it genuinely requires. The Specialist levels go one step further: the puzzle is built so that exactly one advanced technique must be used exactly once.

A candidate is a digit that can still go in an empty cell. Cells see each other when they share a row, column or 3×3 box. A strong link means a digit has exactly two possible cells in one such group: exactly one must contain it.

Specialist

Specialist contains advanced, candidate-based techniques. Each page explains the logic, shows it on a real Sudokuno puzzle and gives you a puzzle to practise on.

Recommended learning order

S1-S10 is a learning progression, not a ranking of technique families. It is the order in which Sudokuno recommends meeting the ten Specialist techniques.

The progression weighs four things:

  • how easy the idea is to understand
  • how easy the pattern is to recognise on a fully pencilled board
  • how difficult the technique is to apply correctly
  • how naturally it builds on concepts introduced earlier

Families and learning order answer different questions. You do not need to finish one family before moving to the next step.

  • X-Wing S1

    An X-Wing appears when one candidate has exactly two possible cells in each of two different rows, and both pairs sit in the same two columns. The four cells form a rectangle. The candidate must then occupy two opposite corners, one per row, so it can be removed from every other cell in those two columns.

  • Y-Wing S2

    A Y-Wing uses three cells with two candidates each. One cell, the pivot, sees both wings. Whichever digit fills the pivot, one wing must hold the digit shared by the wings, so any cell seeing both wings loses that candidate.

  • Chute Remote Pairs S3

    Chute Remote Pairs uses two cells with the same candidate pair in one band of three boxes or one stack of three boxes. A third group of cells, the bridge, rules out both endpoints holding one digit. At least one endpoint must hold the other, so cells seeing both lose that other digit.

  • Unique Rectangles S4

    Unique Rectangles uses the fact that a puzzle has exactly one solution. Four cells in two rows, two columns and exactly two boxes cannot all be restricted to the same candidate pair. At least one corner must use an extra digit, which can rule out candidates.

  • Swordfish S5

    Work on one digit. Find three rows with two or three possible cells each for it, all within the same three columns. Those columns must contain the digit in the selected rows, so remove it from their other cells. The same pattern works with rows and columns reversed.

  • XYZ-Wing S6

    An XYZ-Wing has a pivot with three candidates and two wings with two each. The pivot sees both wings, whose pairs come from its three candidates. The digit shared by the wings must fill one of the three cells, so any other cell seeing all three loses it.

  • Simple Colouring S7

    Simple Colouring follows one digit through a network of strong links. Colour linked cells alternately: either every cell of colour A holds the digit and none of B does, or the reverse. Those two possibilities can rule out candidates.

  • W-Wing S8

    A W-Wing connects two cells holding the same candidate pair through a strong link on one digit. This proves that at least one of the pair cells holds the other digit. Remove that other digit from any cell seeing both pair cells.

  • Rectangle Elimination S9

    Rectangle Elimination tests whether a candidate would leave a box with no place for a digit. Assuming the target contains the digit forces another cell to contain it too. If together they rule out every place in the opposite box, remove the candidate from the target.

  • BUG+1 S10

    BUG+1 applies when exactly one unsolved cell has three candidates and all the others have two, with a specific count pattern across the whole board. One candidate appears three times in that cell's row, column and box. In a puzzle known to have one solution, place that candidate.

Technique families

A family groups techniques by the kind of logic they use. The boundaries are useful, but not absolute: some techniques draw on more than one family.

Fish

Fish confine one candidate across matching rows and columns.

Swordfish is the natural larger extension of X-Wing. Its core idea is not necessarily much harder, but finding a three-row or three-column fish on a full candidate grid is substantially more demanding. That is why it appears at S5 rather than directly after X-Wing.

Wings

Wing techniques combine small candidate sets so that different possibilities lead to the same elimination.

W-Wing overlaps Wings and Chains. It is filed under Wings because the pattern is most naturally introduced through its matching two-candidate wing cells. The strong link connecting them is chain logic, so both viewpoints matter.

Chains

A link is one logical relationship between candidates; a chain is a sequence or network of links. Simple Colouring is the clearest example: alternating strong links form one connected network whose consequences can be followed.

Rectangle Elimination belongs here because its proof follows links and implications between candidates. The word ‘Rectangle’ in its name describes part of the geometry, not the family of its reasoning.

Uniqueness

Uniqueness techniques use the requirement that a well-formed Sudoku has exactly one solution.

Unique Rectangles is S4 because its deadly-pattern idea is local and fairly concrete. BUG+1 is S10: once seen, the move can be simple, but the explanation is more abstract. You must understand the candidate counts across the whole board and why the extra candidate prevents a non-unique Bivalue Universal Grave.

The Foundations

These techniques underpin every puzzle in Sudokuno, and they keep appearing inside the Specialist puzzles too. They do not have pages of their own yet.

R1 - Naked single
Only one digit can go in a cell. Place it.
R2 - Hidden single
A missing digit has only one possible cell in a row, column or box. Place it there.
A1 - Naked pair
Two cells in the same row, column or box contain only the same two candidates. Remove those candidates from the other cells in that group.
A2 - Hidden pair
In one row, column or box, two digits can go only in the same two cells. Remove other candidates from those two cells.
A3 - Point & reduction
If a box can place a digit only in one row or column, remove it from that line outside the box. If a line can place a digit only in one box, remove it from the rest of that box.
E1 - Naked triple
Three cells in one row, column or box contain only three different candidates between them. Each cell may have two or three. Remove those digits from the other cells in that group.
E2 - Point & reduction
The same box/line deduction as A3, with three possible cells for the digit.
E3 - Hidden triple
In one row, column or box, three digits can go only in the same three cells. Remove other candidates from those cells.