SudoLoom Learning System

Sudoku Solving Strategies – From Beginner to Expert

All 6 human solving techniques below are built directly into SudoLoom’s DifficultyGrader & Smart Hint engine. Whenever you click Hint during a game, the solver applies the simplest applicable technique and explains it step by step.

1. Naked Single (Sole Candidate)

Level: Easy

The most fundamental technique: An empty cell has only 1 valid digit remaining because the other 8 digits already appear in its row, column, or 3×3 box.

When to use: Used continuously throughout every game. 100% of Easy puzzles on SudoLoom can be solved using Naked Singles alone.

2. Hidden Single (Unique Candidate in Unit)

Level: Medium

A cell may have several pencil-mark candidates, but across its entire Row, Column, or 3×3 Box, a particular digit X can only fit into that single cell.

When to use: When no Naked Singles remain. Scan digits 1–9 across 3×3 boxes to spot isolated placements.

3. Naked Pair & Hidden Pair (Locked Pairs)

Level: Hard

When 2 cells in the same unit contain only the candidate pair {a, b}, those two digits are locked into those two cells. You can eliminate {a, b} from all other cells in that unit.

When to use: Essential for pruning candidate notes in Hard puzzles to uncover fresh Naked or Hidden Singles.

4. Pointing Pair / Box-Line Reduction

Level: Hard

If all possible positions for digit X inside a 3×3 box lie on a single Row (or Column), then X can be eliminated from the rest of that Row (or Column) outside the box.

When to use: Extremely effective when combined with Snyder Notation (only marking candidates that appear twice in a 3×3 box).

5. X-Wing (Cross-Locked Rectangle)

Level: Expert

When candidate X appears in the exact same 2 columns across 2 different rows (forming 4 corners of a rectangle), X must occupy two opposite corners. All other cells in those 2 columns cannot contain X.

When to use: A core Expert technique to break through deadlocks without guessing.

6. XY-Wing (Three-Cell Pivot Logic)

Level: Expert

A Pivot cell with candidates {X, Y} sees two Wing cells with {X, Z} and {Y, Z}. Regardless of whether the Pivot is X or Y, one of the Wings must be Z. Any cell that sees both Wings can eliminate candidate Z.

When to use: Used in Expert boards rich in bi-value cells (cells containing exactly 2 candidates).

Practice Live with Smart Hints on the Board

Open a Hard puzzle and click Hint to watch the engine highlight candidate pairs visually.