Sudoku
Settling every empty cell with logic instead of guesswork, from singles to chains and puzzle design.
Sudoku is a logic puzzle where the given digits force every remaining cell to a single answer. It starts from one constraint: no digit repeats in a row, a column, or a 3x3 box. From there your toolkit grows, from scanning by eye to pencil marks, to patterns that read two rows against two columns, to chains that follow an assumption to its end. Difficulty comes from the highest technique a puzzle demands, not from how long you took. The higher you go, the more your seat shifts from solver to setter.
Level Overview
| Level | Stage | Hashtags |
|---|---|---|
| Lv.1 | Rules and Singles (HoDoKu Easy) | #SudokuRules#NakedSingle#FullHouse#GridScanning |
| Lv.2 | Unaided Finish (HoDoKu Easy) | #HiddenSingle#Crosshatching#EliminationScan#UnaidedSolve |
| Lv.3 | Candidate Control (HoDoKu Medium) | #PencilMarks#LockedCandidates#NakedPair#HiddenPair |
| Lv.4 | Crossing Patterns (HoDoKu Hard) | #NakedTriple#HiddenTriple#XWing#SpeedSolving |
| Lv.5 | Chain Reasoning (HoDoKu Unfair) | #Swordfish#XYWing#ChainLogic#SudokuGrandPrix |
| Lv.6 | Construction and Verification (HoDoKu Extreme) | #ForcingChain#AlmostLockedSets#PuzzleConstruction#DifficultyGrading |
| Lv.7 | Technique Creation | #TechniqueCanon#VariantInvention#RatingStandard#SudokuPedagogy |
Levels
Level 1
#SudokuRules#NakedSingle#FullHouse#GridScanningLearning the rules and filling cells where only one candidate fits.
You've just stepped into sudoku. You pick up the constraint that no digit repeats in a row, a column, or a 3x3 box. Then you fill the spots where only one cell is left in a unit. You also fill cells whose candidates narrow to one, a naked single. On the technique repertoire axis you use the lowest tier only. On the search cost axis you sweep the whole grid over and over because you don't know where to look. Cover the hints and the next move stops coming.
What Comes Next
If you've checked off most of this list, you're ready to enter the Lv.2 Unaided Finish stage and take on closing a puzzle with no hints. HoDoKu's technique ranking puts hidden singles right after naked singles, which flips the question you ask from cell first to digit first. Pick one digit at a time from 1 to 9 and sweep the whole grid for that digit alone. Do it in short repeated bursts and the flip settles in before anything else.
Level 2
#HiddenSingle#Crosshatching#EliminationScan#UnaidedSolveFinishing easy puzzles unaided through crosshatching.
Your first independence shows up here: you close a puzzle with no hints. You find the spots where a digit has only one home inside a line or a box, called a hidden single. Crosshatching two lines against each other narrows its position inside a box. On the search cost axis your sweeps gain an order, and the same puzzle takes noticeably less time. You still carry it all by eye, with no candidates written down.
What Comes Next
If you've checked off most of this list, you're ready to enter the Lv.3 Candidate Control stage and take on solving by writing candidates down and erasing them. Chase & Simon(1973) showed that skilled chess players read a board as meaningful units rather than separate pieces. The same shift in perception takes time in sudoku scanning, and until it lands, writing candidates on the grid is faster. Pick 10 puzzles your eyes alone can't close and write out every candidate.
Level 3
#PencilMarks#LockedCandidates#NakedPair#HiddenPairTracking pencil marks to clear medium puzzles without guessing.
This is where dependence turns into autonomy. You keep pencil marks, writing every possible digit into every empty cell. Then you clear the cells around them with locked candidates, where a box's candidates for one digit all sit in a single line. Naked pairs, two digits trapped in two cells, do the same work. On the technique repertoire axis your tools move from filling in to erasing, and medium puzzles close with no guessing.
What Comes Next
If you've checked off most of this list, you're ready to enter the Lv.4 Crossing Patterns stage and take on eliminations that read two rows against two columns. HoDoKu's table puts X-Wing after pairs and triples because the stage your reasoning plays on widens from inside one line to the crossing of rows and columns. Solve the same puzzle twice and cut your pencil marks in half on the second pass, and your view widens from the line to the grid.
Level 4
#NakedTriple#HiddenTriple#XWing#SpeedSolvingOpening hard puzzles with triples and X-Wing.
You stop running a set procedure and start picking tools by reading the position. Naked triples, three digits locked in three cells, and X-Wing, built from two rows and two columns, open puzzles at the hard grade. The speed axis enters for the first time: you time your finishes at one difficulty and manage the spread. You also judge the point where scanning by eye has nothing left to give and switch over to pencil marks.
What Comes Next
If you've checked off most of this list, you're ready to enter the Lv.5 Chain Reasoning stage and take on following an assumption to its contradiction. The Sudoku Grand Prix packs a round into 90 minutes, so when you drop a puzzle counts toward your score. Ericsson(1993) found that gains come from repetition aimed at weaknesses, not from comfortable practice. Collect the puzzles that stopped you, solve 20 in a row needing the same technique, and note where you stall.
Level 5
#Swordfish#XYWing#ChainLogic#SudokuGrandPrixRunning chain logic and competing in timed rounds.
Your reach widens from your own solving to the people around you. Swordfish across three rows and three columns, XY-Wing linking three cells, and simple coloring bring you into chain reasoning. Coloring splits one digit's candidates into two colors until one side contradicts itself. You learn to judge which puzzles to take and which to leave in a timed international round, and you coach people who have just started, step by step.
What Comes Next
If you've checked off most of this list, you're ready to enter the Lv.6 Construction and Verification stage and take on building puzzles that hold exactly one answer. HoDoKu covers solving techniques and puzzle generation in one document because the two jobs run the same knowledge in opposite directions. Take a puzzle you solved, pull the clues out one at a time, and watch for the point where difficulty climbs a step. That experiment opens the setter's eye.
Level 6
#ForcingChain#AlmostLockedSets#PuzzleConstruction#DifficultyGradingWorking forcing chains and building puzzles with one answer.
Your seat moves from solver to setter. Almost locked sets and forcing chains carry you through the hardest puzzles with no guessing, and you leave the path behind in a form other people can retrace. The construction and verification axis opens: you build puzzles whose single answer is guaranteed, and you grade their difficulty by the hardest technique they demand.
What Comes Next
If you've checked off most of this list, you're ready to enter the Lv.7 Technique Creation stage and take on widening the technique list. HoDoKu's grades stop here, and so do the Grand Prix rules. Ericsson(1993) set immediate feedback as a condition of deliberate practice, and once the fixed sequence ends, you have to arrange it yourself. Write your method up as a named procedure, give the same puzzle to three or more solvers, and collect the points where they split.
Level 7
#TechniqueCanon#VariantInvention#RatingStandard#SudokuPedagogyLeaving techniques and variants that later solvers take as standard.
You leave a mark on how sudoku itself is spoken about. Other communities adopt the technique or notation you wrote up exactly as it stands. The variant rules you designed settle in as a competition category or a regular slot on a setting list. No technique list and no competition rulebook names a step beyond here, so the evidence of arrival is only how long the methods you left keep being cited.
What Comes Next
If you've checked off most of this list, you stand where you leave behind the language that explains sudoku rather than one more way to solve it. Standings change every year, but named techniques and grading standards stay, and they decide what someone facing a grid for the first time looks at first. What this stage produces shows up in the puzzles other people solve with that language, not in the ones you solved yourself.
References
Surveys the metrics used to rate sudoku difficulty and tests how well they correlate with human solving time, giving the academic grounding for level boundaries drawn at the hardest technique a puzzle demands.
Ranks solving techniques by difficulty, from naked and hidden singles to naked and pointing pairs, then X-Wing, then Swordfish, XY-Wing and chains, so the order in which you pick them up becomes the level boundaries.
The international federation runs eight 90-minute rounds and ranks solvers on their best six, an official competition system that gives the top levels a real destination and a speed benchmark.
Chase & Simon(1973) is the classic study showing that skilled chess players read a board as meaningful units rather than separate elements, grounding the Lv.2 to Lv.4 checklist items and transition hints about scanning efficiency and technique recognition speed.
Ericsson(1993) concluded that skill comes from repetition aimed at weaknesses with immediate feedback, not from accumulated hours, grounding the transition hints that prescribe what to repeat at each level and how.