Learn Sudoku
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When a cell has only one number left that it can legally hold, that number must go there. This is the most fundamental deduction in Sudoku.

Within a single row, column, or box, a digit sometimes has only one cell where it can legally live — even if that cell has other candidates too.

Scanning is the habit of sweeping your eyes across rows, columns, and boxes to spot where a digit can and cannot go — the foundation every other technique builds on.

A disciplined form of scanning: for one target box, use the rows and columns passing through it to eliminate cells until a digit has just one home.

Rather than asking “what goes here?”, elimination asks “what cannot go here?” Removing impossible candidates is what eventually exposes every single.

When a digit's only possible cells inside a box all sit on one line, or a line's only cells sit inside one box, that digit is 'locked' — letting you eliminate it elsewhere.

A box-to-line variant of locked candidates: two or three cells in a box that share a candidate and lie on the same line 'point' outward, eliminating that candidate along the line.

The mirror image of pointing pairs. When a candidate within a row or column only appears inside one box, it can be cleared from the remaining cells of that box.

Two cells in the same house that both hold exactly the same two candidates. Those two digits are reserved for those two cells, so they can be removed from every other cell in the house.

Two digits that, within a house, can only appear in the same two cells — even though those cells carry other candidates too. Those extra candidates can be cleared.

Three cells in a house that collectively use only three candidates. The three digits are locked to those cells and can be removed from the rest of the house.

Three digits confined to the same three cells of a house, hidden among other candidates. The other candidates in those cells can be removed.

A rectangle pattern. When a candidate appears in exactly two cells on each of two rows, and those cells line up in the same two columns, the candidate is eliminated from the rest of those columns.

An X-Wing scaled to three rows and three columns. A candidate confined to at most three cells in each of three rows, all within the same three columns, is eliminated elsewhere in those columns.

The four-by-four extension of the X-Wing / Swordfish family. A candidate confined to four cells across four rows, aligned within four columns, is eliminated from the rest of those columns.

A three-cell chain. A 'pivot' cell with candidates {X,Y} sees two 'pincer' cells {X,Z} and {Y,Z}. Whatever the pivot becomes, one pincer must be Z — so Z is eliminated from cells both pincers see.

An XY-Wing whose pivot carries three candidates {X,Y,Z}. Because the pivot itself could be Z, eliminations only apply to cells seen by all three of the pivot and both pincers.

Exploits the guarantee that a proper puzzle has exactly one solution. Four cells forming a rectangle across two boxes with the same two candidates would create two solutions — so that 'deadly pattern' must be broken.

For a single candidate, chain together cells linked by 'conjugate pairs' (houses where the candidate has only two spots) and alternately colour them. Contradictions in the colouring drive eliminations.

Sequences of strong and weak links in a single candidate. Following the chain of 'if this is off, that is on' yields eliminations at the ends where both endpoints see a common cell.

Chains that close back on themselves. A continuous alternating loop enforces eliminations on every weak link around it, often clearing several candidates at once.

The nuclear option short of guessing. Assume a candidate in a cell, follow the forced consequences, and if every possibility for that cell leads to the same conclusion, that conclusion is certain.