Cross-hatching
First needed on EasyFor each digit, scan the rows and columns that already contain it to figure out which cell in a target box must hold that digit. The first technique most beginners learn.
Beginner tips →Every solving technique in roughly the order you should learn them. Foundations on easy puzzles, fancier moves higher up. The difficulty band on each is the lowest tier where that technique first becomes necessary.
Without these you can't finish a sudoku at any difficulty. Built entirely on visual scanning + the constraint that each digit appears once per row/column/box.
For each digit, scan the rows and columns that already contain it to figure out which cell in a target box must hold that digit. The first technique most beginners learn.
Beginner tips →A cell with only one legal candidate given the rest of the board. Place it. Most Easy puzzles fall entirely to cross-hatching + naked singles.
Practice naked singles →Once foundations stall, you fill in pencil-mark candidates and start reasoning about which digits go where rather than which cells need filling. These techniques are mechanical enough to scan for.
If a digit's candidates in a 3×3 box are all in the same row (or column), the digit must end up in that row inside the box — so it can't appear elsewhere in the row outside the box.
Pointing pairs guide →The mirror of pointing pairs. If a digit can only go in cells of a single box within a row, the digit can't appear elsewhere in the box outside the row.
Pointing pairs + box-line →Two cells in a unit share exactly two candidates → those two digits are locked into the pair → erase both from elsewhere in the unit. Extends to triples with three cells covering three digits.
Naked pairs + triples →From here on, you're reasoning about patterns across multiple rows or columns simultaneously. Mid-Expert puzzles routinely require these.
A digit restricted to two cells in each of two rows, sharing the same column pair (a rectangle). Eliminates the digit from the rest of those two columns.
X-wing guide →Three bivalue cells in a chain: pivot {A,B}, wings {A,C} and {B,C}. The shared candidate C is eliminated from any cell that sees both wings.
Y-wing guide →Y-wing's trivalue-pivot variant. Pivot {x,y,z}, wings {x,z} and {y,z}. Any cell that sees all three loses z as a candidate. Rarer than Y-wing but real on Expert+.
XYZ-wing guide →Two strong links on the same digit, sharing a single column/row. The non-shared ends eliminate the digit from cells that see both. Easier to spot than X-wing in many cases.
Skyscraper guide →The 3-row extension of X-wing. A digit confined to 2-3 cells across each of three rows, all within the same three columns. Eliminates that digit from the rest of those columns.
Swordfish + jellyfish →The 4-row extension. Rare in published puzzles; when it appears, the cross-row analysis is what unlocks it.
Swordfish + jellyfish →These last few apply when nothing simpler works. On Extreme puzzles you might need exactly one of these to break the impasse; spotting them takes practice.
Four cells in a rectangle across two rows/columns/boxes that share two candidates would create two valid solutions if undisturbed. Since the puzzle has one solution, an extra-candidate cell forces an elimination.
Unique rectangle guide →A chain of bivalue cells where each link forces the next. The endpoints have a common candidate that can be eliminated from any cell that sees both. Y-wing is the simplest XY-chain.
If both candidates of a bivalue cell lead to the same conclusion elsewhere, that conclusion holds. Trial-and-error promoted to legitimate technique.
Generalization of naked subsets where n cells share n+1 candidates. Combining two ALSs that share a common digit forces eliminations on a third digit. The frontier of human solving.