Sudoku guides

Straight answers about the parts of sudoku that are genuinely hard to explain. Written to be useful whether or not you ever install the app.

• What an X-Wing means in sudoku

  The pattern, why the elimination is forced rather than likely, how to spot it without hunting, and how Swordfish is the same idea one size larger.

• Naked pairs and hidden pairs, explained

  What a naked pair removes, what a hidden pair removes, why they are opposites, and the mistake that makes players think a hidden pair does nothing.

• Why does my sudoku have more than one solution?

  The four real causes, the rectangle pattern that guarantees a second solution, why 17 is the famous number, and what a solver actually checks.

• Pointing pairs and claiming, explained

  The two box–line interactions: when a box pins a digit to one line, and when a line pins a digit to one box. Cheap, constant, and the reason most Medium puzzles fall.

• What an XY-Wing is, and how to find one

  A pivot with two candidates and two pincers, why the elimination holds whichever way the pivot falls, and the search that finds one in a minute instead of an hour.

• How to keep pencil marks from becoming a mess

  When to start marking, why partial marking beats full marking on paper, the maintenance rule that keeps notes trustworthy, and what changes when the app does the bookkeeping.

• Stuck on a sudoku: what to check, in order

  A fixed escalation order from cheapest to most expensive, why running it in order matters more than knowing more techniques, and how to tell being stuck from having already gone wrong.

• How sudoku difficulty is actually decided

  Why clue count is a bad measure, what grading by hardest required technique means, and why two puzzles with the same number of givens can be worlds apart.

• Is guessing allowed in sudoku?

  What a computer means by guessing, why it is not the same thing a person does, when a case split is legitimate reasoning, and what it means for a puzzle to require trial and error.

• Is sudoku good for your brain? An honest answer

  What the research on brain training actually shows, why transfer effects are weak, where the name came from, and the reasons to play that survive scrutiny.