The Sudoku Slate guidebook
Learn Sudoku.
See why a move works.
Make sense of candidates, learn why an elimination works, and practise on a different position. Follow a technique from its exact condition to the evidence in our puzzle catalog.
Beyond singles
Learn what you can remove.
Locked candidates
One intersection. Two directions of reasoning.
Worked position + new practice →
Naked pairs
Two cells reserve two digits.
Worked position + new practice →
Hidden pairs
Follow two digits to their only two cells.
Worked position + new practice →
Build a reason for every move
Start with candidates, then learn two ways to turn possibilities into a proven placement.
Sudoku candidates: how to make pencil marks that help
Calculate Sudoku candidates from the row, column and box. Work through r1c1 = {1, 4}, then see why a legal candidate is not a proven answer.
Naked singles in Sudoku: one cell, one possible digit
Learn the naked single with a coordinate-labelled Sudoku example. Combine row, column and box restrictions to prove r1c1 = 1 without guessing.
Hidden singles in Sudoku: find the only place for a digit
See why r1c2 must be 2 even though its candidates are {1, 2}. A worked row example explains hidden singles, complete scans and common mistakes.
Understand what went wrong
Repeated digits, an impossible position and an ambiguous puzzle are different problems. Learn to tell them apart.
Sudoku conflicts: find repeated digits without blaming the wrong cell
Diagnose duplicate digits in rows, columns and boxes with a fixed example. Learn why a conflict identifies a contradiction, not which entry to erase.
Can Sudoku have no solution without any repeated digits?
Yes: a conflict-free Sudoku can still be impossible. Follow a concrete contradiction in row 1 and learn how to distinguish an invalid board from an unsatisfiable one.
Can a Sudoku have multiple solutions? A concrete example
A 63-clue Sudoku can still have multiple solutions. See two complete grids that preserve every given, and learn what a uniqueness check really proves.