Sudoku solving techniques, ranked by difficulty

Most technique lists are alphabetical or arbitrary. This one is ordered the way it matters: by the difficulty tier each technique signals. A puzzle only forces a technique when nothing cheaper works, so the hardest technique you're made to use is a direct readout of how hard the puzzle is. Learn them bottom-up, in the order a fast solver actually reaches for them.

Tier 1 — singles: the moves you make constantly

Singles place a digit directly. They're the bulk of every solve, and making them instant is the highest-leverage thing you can do for speed.

Open single (last cell)

A box, row, or column with eight cells filled has exactly one empty cell, and only one digit is missing from that unit. Place it. The easiest move in sudoku, and worth a glance after every placement, because filling one cell often completes a unit somewhere else.

Hidden single

A digit that can legally go in only one cell of a unit, even if that cell shows other candidates. Every other cell in the unit is blocked from taking that digit by a clash in its row, column, or box, so this cell must take it. Hidden singles are the engine of fast solving — scan digit by digit across the boxes and most of your placements will be these. Full guide to hidden singles →

Naked single

A cell that has only one candidate left — every other digit is ruled out by its row, column, and box. Certain, but harder to spot by eye than a hidden single, so you catch these on a row-and-column sweep or when candidate marks are visible.

Tier 2 — pairs and locked candidates: your first eliminations

These don't place a digit. They remove candidates, which unlocks the next single. Learning to see them is the jump from beginner to intermediate.

Naked pair

Two cells in the same unit that each contain exactly the same two candidates and nothing else — say both show only {3, 7}. Those two cells will use up 3 and 7 between them, so 3 and 7 can be erased from every other cell in that unit. You don't know which is which yet, and you don't need to. Full guide to naked pairs →

Hidden pair

The mirror image: two digits that can only appear in the same two cells of a unit, even though those cells also list other candidates. Those two cells must hold those two digits, so every other candidate is erased from them. Harder to spot than a naked pair because the pair is hiding among extra candidates.

Pointing pair and claiming (locked candidates)

When a digit's only possible cells inside a box all lie on one row or column, the digit must come from that line within the box, so it can be erased from the rest of that line outside the box — that's a pointing pair. Claiming is the reverse: when a digit in a row or column is confined to a single box, erase it from the rest of that box. Cheap, extremely common, and constantly missed. Full guide to pointing pairs →

Tier 3 — triples and the X-Wing: stop-and-scan territory

Here singles-clicking stops working; you have to deliberately look for structure. A puzzle that requires T3 is a solid "hard."

Naked and hidden triples

The same logic as pairs, extended to three cells and three digits. Three cells in a unit collectively holding only three candidates (naked), or three digits confined to the same three cells (hidden), let you eliminate accordingly. The candidates don't all have to appear in every cell — the three digits just have to be contained across the three cells.

X-Wing

A single-digit pattern. When a digit appears in exactly two cells of one row and exactly two cells of another row, and those four cells share the same two columns, they form a rectangle. The digit sits on one diagonal or the other, which means each of those two columns gets the digit from inside the rectangle — so you can erase it from every other cell in those two columns. It also works with rows and columns swapped. The first "advanced" pattern most solvers learn. Full guide to the X-Wing →

Tier 4 — wings and quads: genuinely advanced

Rare in everyday puzzles. Learn these for completeness — so a hard grid can't stop you cold — not because they'll speed up your average solve.

Skyscraper (and 2-string kite)

Single-digit chain patterns built from two strong links. A skyscraper uses a digit that's confined to two cells in each of two lines, where the lines share one "base"; any cell that sees both far ends of the pattern can't hold the digit. A compact, high-value pattern once your eye is trained for it.

XY-wing and XYZ-wing

An XY-wing uses a pivot cell with candidates {X, Y} that sees two cells holding {X, Z} and {Y, Z}. Whichever digit the pivot takes, one wing becomes Z, so any cell seeing both wings can't be Z. The XYZ-wing is the same idea with the pivot itself also holding Z, which tightens which cells the elimination reaches.

Naked and hidden quads

Pairs and triples taken one step further: four cells holding four candidates between them, or four digits confined to four cells. Valid but uncommon, and usually there's a cheaper move available if you keep scanning.

Tier 5 — fish and uniqueness: the ceiling

The hardest techniques Speedoku's engine ever requires. Beyond here lie chains and colouring, which we deliberately leave out — they fight the speed identity of the game.

Swordfish

An X-Wing scaled from two lines to three. A digit is confined to at most three cells in each of three rows, and those cells occupy only three columns; the digit can then be erased from those three columns everywhere outside the pattern. Hard to spot, satisfying to land.

Unique rectangle

A uniqueness technique. It exploits the fact that a proper sudoku has exactly one solution: a certain rectangle of the same two candidates in two boxes would create two valid solutions, which is impossible, so a candidate can be eliminated to avoid that "deadly pattern." Powerful, and a reminder that a well-made puzzle's single solution is itself information.

How to use this list

Learn top-down through the tiers, and don't rush the jump. Nearly all of your solving happens in T1 and T2, so that's where speed is won — see the guide to getting faster for the drills. Treat T3 and up as difficulty markers first and speed tools second: when a puzzle forces one, you've learned something about the puzzle, not just made a move.