Sudoku is a pure logic puzzle. Despite the grid of numbers, no arithmetic is involved — and, crucially, no luck. A well-made puzzle has exactly one solution that can be reached by reasoning alone, with no point where you are forced to guess and hope. EveryCardHearth Sudoku is guaranteed to be like this: one unique answer, solvable start to finish by deduction. So if you ever feel like guessing, the right response is to find the logical step you have missed. This guide gives you the ladder of techniques to do exactly that, in the order you will actually reach for them.
The foundation: pencil marks
Almost every technique below rests on candidates — the small "pencil marks" you note in a cell to record which digits could still legally go there. A candidate survives only if that digit does not already appear in the cell's row, its column, or its 3×3 box. Once a cell's candidate list is accurate, the puzzle starts to reveal its own structure: cells with a single candidate, digits with a single home, and patterns that let you cross out candidates elsewhere. On CardHearth you can toggle candidates on; even when you fill them in by hand, the habit of thinking in candidates is what turns a hard puzzle from "stuck" into "solvable."
The technique ladder
1. Naked single
A naked single is the simplest move there is: a cell whose candidate list has been whittled down to exactly one digit. If a cell can only be a 7, write the 7. Then update the candidates in that cell's row, column, and box — filling one cell often creates the next naked single, and a chain of them can carry you a long way. Use this whenever you have just placed a digit or eliminated candidates.
2. Hidden single
A hidden single is the most important everyday technique. Instead of asking "what can go in this cell?", you ask "where can this digit go in this unit?" — where a unit is a row, a column, or a box. If a digit has only one cell left in a unit where it is still a legal candidate, it must go there, even if that cell still shows several other candidates. Scan one digit at a time across each box: if eight of the nine cells are blocked for that digit, the ninth is forced. This single idea solves most of an Easy puzzle and a great deal of harder ones.
3. Locked candidates (pointing and claiming)
Sometimes a digit's candidates inside one box are all confined to a single row or column. That is a locked candidate, and it cuts both ways:
- Pointing: if every spot for a digit within a box lies in one row (or column), that digit must end up in that row inside this box — so you can eliminate it from the rest of that row in the other boxes.
- Claiming: the reverse — if every spot for a digit within a row (or column) lies in a single box, you can eliminate that digit from the other cells of that box.
Neither places a digit directly; both remove candidates, which then unlocks singles. Reach for this when scanning for singles stalls.
4. Naked pairs (and triples)
A naked pair is two cells in the same unit that share the exact same two candidates — say both can only be {4, 9}. You do not yet know which is the 4 and which is the 9, but you know those two digits are used up by those two cells. So you can safely eliminate 4 and 9 from every other cell in that unit. The same logic extends to anaked triple: three cells in a unit that between them use only three candidates (each cell need not show all three) lock those digits to those cells, clearing them elsewhere in the unit. These eliminations frequently expose a hidden single you could not see before.
5. Hidden pairs
A hidden pair is the dual of the naked pair. Here, two digits can each go in only the same two cells of a unit — but those cells also carry other candidates that hide the pattern. Because those two digits must occupy those two cells between them, everyother candidate in those two cells can be deleted. For example, if 2 and 6 can only live in cells A and B of a row, then A and B become a {2, 6} pair and any other candidates in A or B are crossed out. Spotting hidden pairs takes a deliberate digit-by-digit scan of the unit, which is why they tend to show up only on the hardest puzzles.
6. X-Wing
The X-Wing is the first true "fish" pattern, and the toughest technique you will ever need here. Look at a single candidate digit — say 5. Find two rowsin which that digit has exactly two possible cells each, and where those cells fall in thesame two columns. Those four cells form the corners of a rectangle. Whichever way the 5s resolve, one of the two columns gets its 5 from the top row and the other from the bottom row — so in both columns, the 5 is committed to one of these two rows. That means 5 can be eliminated from those two columns everywhere else. The pattern works identically with rows and columns swapped: two columns that confine the digit to the same two rows let you clear it from those rows elsewhere. The X-Wing places nothing on its own; like locked candidates, it is an elimination that breaks a puzzle back open.
How the techniques map to CardHearth difficulties
Our difficulty tiers are defined by exactly which rung of this ladder a puzzle can require — never by adding luck or removing the unique solution.
- Easy — solvable with naked and hidden singles alone. No candidate bookkeeping strictly required.
- Hard — adds locked candidates (pointing and claiming) and naked pairs. You will want pencil marks here.
- Expert — can require hidden pairs and the X-Wing. Still pure logic, just deeper patterns.
Whatever the tier, the guarantee holds: one solution, reachable by deduction. The standardSudoku game mixes these tiers as you play.
Habits of fast solvers
- Scan systematically. Sweep boxes, then rows, then columns for one digit at a time, looking for hidden singles, before you reach for anything fancier.
- Fill candidates only when scanning stalls. Marking up the whole grid too early is slow and error-prone; let the easy placements go first, then pencil in candidates where you are stuck.
- Attack the most-constrained units first. A row, column, or box that is already mostly full is where forced placements hide — start where the grid is densest.
- Never guess. If nothing moves, you have missed an elimination, not run out of logic. Re-scan for locked candidates and pairs; the puzzle always yields.
Frequently asked questions
Do you ever have to guess in Sudoku?
No — not in a fair puzzle. A well-made Sudoku has exactly one solution that can be reached entirely by logic, with no point where you must guess and backtrack. Every CardHearth puzzle is guaranteed to have a unique solution and to be solvable by pure deduction, so if you are tempted to guess, there is always a logical step you have not spotted yet.
What is the difference between a naked single and a hidden single?
A naked single is a cell that has only one candidate left — you look at the cell and see one possibility. A hidden single is a digit that has only one place it can go within a row, column, or box, even though that cell still shows several candidates. Naked singles are about the cell; hidden singles are about the digit. Hidden singles are the most common everyday move in real solving.
What is the hardest technique I need on CardHearth?
The X-Wing, and only on Expert puzzles. Easy puzzles need nothing beyond singles, Hard puzzles add locked candidates and naked pairs, and Expert can require hidden pairs and the X-Wing. You never need anything more exotic, because every puzzle is generated to be solvable by exactly this ladder of techniques.
Keep going
The fastest way to internalise this ladder is to climb it in order: play a Sudoku and consciously name the technique behind each move. When you want a different kind of planning challenge, our how to win Klondike Solitaire guide rewards forward thinking in a very different way, and theguides hub covers the whole CardHearth library.