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.
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.
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.
