Hard Sudoku
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Hard Sudoku - Mastering Logical Deduction
Hard puzzles demand you see patterns across multiple cells simultaneously. The four techniques below form the complete toolkit for this level - Learn them and no Hard puzzle will beat you.
Medium puzzles can always be solved by examining one unit at a time. Hard puzzles force you to look at relationships between units. A digit can't go here - Not because of a single row or box, but because of combined constraints from two or three units at once.
- Naked Pair - Two cells in the same row, column or box that each have exactly the same two candidates (e.g. both show {3,7}). Those two digits must occupy those two cells in some order, so they can be removed as candidates from every other cell in that unit.
- Hidden Pair - Two cells in a unit that are the only two cells where two specific digits can appear - Even though those cells may have other candidates too. Remove all other candidates from those two cells, then treat them as a Naked Pair.
- Pointing Pair / Triple - Within a 3×3 box, if a candidate appears in only one row or one column, it must go somewhere in that row or column within the box. You can therefore eliminate it from the rest of that row or column outside the box.
- Box/Line Reduction - The reverse of Pointing Pairs. If a candidate in a row or column appears only within a single 3×3 box, it must be placed in that box, so eliminate it from other cells in that box.
- Write in all candidates for every blank cell using Notes mode.
- Scan each row, column and box for any two cells that share exactly the same two-digit candidate set.
- When found: those two digits are locked to those two cells - Remove them from all other cells in the shared unit.
- This often reveals Naked Singles or new pairs elsewhere; cascade through the board.
- Re-run all Easy and Medium techniques after each elimination - Pairs frequently unlock simpler moves.
- Fill all candidates in Notes mode before attempting advanced techniques
- Exhaust Naked Singles and Hidden Singles first
- Scan all rows, columns and boxes for Naked Pairs
- Check each box for Pointing Pairs (confined candidates)
- Apply Box/Line Reduction to any row/column candidate confined to one box
- After each elimination, return to step 2 - New pairs appear as the grid fills
- Never guess - All Hard puzzles yield to pure logic
Proportion of moves per technique on average:
| Solver Level | Typical Time | Common Blocker |
|---|---|---|
| New to Hard | 45–90 min | Not recognising pairs |
| Comfortable | 20–40 min | Missing Pointing Pairs |
| Experienced | 10–20 min | - |
| Speed solver | < 8 min | - |
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