Sudoku Techniques: Trial and Error (for Demon, Hell and Abyss)
When no other technique makes progress, assume a number and look for a contradiction.
What is the trial-and-error method?
Pick a cell with only two candidates, tentatively place one of them, and play it out. If you reach a point where the same number appears twice, or a cell is left with no possible number, the assumption was wrong. In that case, the other number is the answer.
Tips for doing it
Write the assumed number, and the numbers you place as a result, so they can be told apart from the originals, for example in a different color or in erasable pencil. If a contradiction appears, erase everything you placed from that assumption.
Use other techniques first
Trial and error takes effort, so use it only after you have exhausted singles, pairs, pointing pairs, X-Wing and the like.
Demon, Hell and Abyss
On this site, Demon, Hell and Abyss are puzzles where logical techniques alone leave you stuck and trial and error is needed. They have 24–27 clues in Demon, 21–24 clues in Hell, and 19–21 clues in Abyss.
More articles: Sudoku Rules and How to Get Started / The Two Basic Sudoku Techniques: Singles (One Number per Cell, One Cell per Number) / How to Use Notes (Candidates) in Sudoku / Sudoku Techniques: Pairs (Naked Pair and Hidden Pair) / Sudoku Techniques: Boxes and Lines (Pointing Pairs) / Sudoku Techniques: X-Wing / How Kids Can Play Sudoku (6×6)
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