Sudoku Techniques – From Naked Singles to X-Wing

Two squares show the same two notes. Together they reserve both digits for themselves.

Two digits have only the same two homes in a unit. Every other note in those cells can go.

Three squares reserve three digits between them, so those digits can be crossed out elsewhere in the unit.

Three digits have only the same three homes. Remove every other note from those cells.

Four squares share only four candidate digits; remove those digits from every other square in that unit.

Four digits have only four homes. Clear every other note from those homes.

Two matching lines give one digit exactly the same two possible homes. That rectangle removes the digit from the rest of those crossing lines.

The three-line version of the X-Wing: a digit confined to three rows and three columns.

The four-line fish. A digit restricted to four rows and four columns clears those columns everywhere else.

A Fish pattern with one extra candidate — the fin — whose box makes a local elimination safe.

A Fish with one missing base corner and a fin in the same box, producing a precise local elimination.

Two lines sharing a base but with tops in different columns. Cells seeing both tops can drop the digit.

A row and a column, each with two candidates, meeting through a shared box. Their far ends cannot both be false.

A box where a digit's candidates form a cross rather than a rectangle, combined with a strong link elsewhere.

Follow one digit's strong links across the board with two alternating colours, then look for contradictions.

An alternating chain for one digit: strong and weak links ensure that at least one end is true, so a cell seeing both ends loses that digit.

A chain of cells all holding the same two candidates. The ends behave like a pair even when far apart.

A longer chain of bi-value cells. Linked pairs force a shared endpoint digit, which can be eliminated from cells seeing both ends.

A pattern that would create two solutions. Since a proper puzzle has only one, it can be ruled out.

Two adjacent rectangle corners share one extra candidate. One must take it, so cells seeing both lose it.

When every unsolved cell but one has two candidates, the odd cell out must take its third digit.

Assume a candidate, follow the consequences, and see whether both assumptions lead to the same result.

A short chain of strong and weak links whose matching endpoints eliminate a candidate.

A closed alternating chain that proves a candidate false, true, or impossible outside the loop.

Two separate colouring components for one digit collide, forcing their opposite colours and removing shared targets.

Two nearly locked candidate groups share a restricted digit, allowing a second shared digit to be eliminated outside both sets.

Three Almost Locked Sets are joined by two different restricted common candidates, forcing an outside candidate from both outer sets.