Learn sudoku
Twenty short guides, in the order you will need them. Each one says which level on sudoku.ie it unlocks. The hint button in the game links straight to the guide for the step it shows you.
Getting started
- Sudoku basicsThe rules of sudoku in plain English, how the grid is laid out, and a simple way to make your first moves on any puzzle.
- Scanning and cross-hatchingCross-hatching is the quickest way to place digits: use the rows and columns around a box to rule out cells until only one is left.
- Pencil marks and notesHow to note the possible digits in each cell, keep those notes tidy, and use them to find moves you cannot see by scanning alone.
- What to do when you are stuckA calm, step-by-step routine for when a puzzle stops giving: check your work, tidy your notes, then work through the techniques in order.
- How to solve fasterPractical habits that cut minutes off your times: scan in a fixed order, delay full notes, follow chains of placements and avoid costly mistakes.
Easy techniques
Medium techniques
- Pointing pairs and triplesWhen a digit's candidates in a box all sit in one row or column, that digit can be removed from the rest of that row or column.
- ClaimingWhen a digit's candidates in a row or column all fall inside one box, that digit can be removed from the rest of that box.
- Naked pairsTwo cells in a row, column or box with the same two candidates lock those digits, so they can be removed from every other cell there.
- Hidden pairsWhen two digits can only go in the same two cells of a row, column or box, every other candidate can be cleared from those two cells.
Hard techniques
- Naked triplesThree cells in one row, column or box whose candidates use only three digits between them lock those digits away from every other cell.
- Hidden triplesWhen three digits can only go in the same three cells of a row, column or box, all other candidates can be cleared from those cells.
- X-wingWhen a digit can only go in the same two columns in two different rows, it can be removed from the rest of those two columns.
- Naked quadsFour cells in one row, column or box whose candidates use only four digits between them lock those digits away from the rest of that unit.
- Hidden quadsWhen four digits can only go in the same four cells of a row, column or box, every other candidate can be cleared from those four cells.
Expert techniques
- XY-wingThree two-candidate cells linked in a bent chain let you remove a digit from any cell that can see both ends of the chain.
- XYZ-wingLike an XY-wing, but the pivot has three candidates. A digit shared by all three cells can be removed from cells that see all three.
- SwordfishThe X-wing with three rows and three columns: when a digit is confined to three columns across three rows, clear it from the rest of those columns.
- JellyfishThe four-line version of the X-wing and swordfish. Rare and tricky, but the logic is the same: a digit locked into four rows and four columns.