The two moves that solve most easy puzzles

Most easy Sudoku puzzles are solvable with two specific moves, repeated until the grid fills in. Once you know how to spot them, you stop seeing easy puzzles as a wall of empty cells and start seeing them as a sequence of small obvious next steps. The two moves are the naked single and the hidden single, and the difference between them is mostly a difference of perspective.

If you've just learned the rules and want a refresher, our complete beginner's guide covers the basics. This piece picks up from there with the next thing worth learning.

The naked single

Pick any empty cell on the grid. Look at every digit already placed in its row, its column, and its 3×3 box — anything in any of those three groups can't go in your chosen cell. Cross those digits off mentally. If exactly one digit survives, that's the one that goes in the cell.

That's a naked single: a cell with only one legal candidate left. On easy puzzles, naked singles are common from the start, and as you place each one, more naked singles appear as the constraints tighten elsewhere. A typical easy puzzle solves to roughly 60% complete on naked singles alone before you have to do anything else.

The naked single is the move beginners learn first because it's the one you can find by staring at one cell at a time. You don't need to keep track of the whole grid; just the cell you're looking at.

The hidden single

Now stop staring at cells and start looking at units — entire rows, entire columns, entire 3×3 boxes. Pick a unit and pick a digit that hasn't been placed in it yet, then ask: of all the empty cells in this unit, how many could legally hold this digit? If the answer is exactly one, that cell is where the digit goes — even if the cell could also legally hold other digits.

That's a hidden single. The cell isn't a naked single (more than one digit could go there), but the unit only has one place for that specific digit, so the digit has nowhere else to go. The cell looked ambiguous from a cell-first perspective and unambiguous from a digit-first perspective. Both perspectives have to be available, and you have to be able to flip between them.

Hidden singles are the move that turns a 60%-solved easy puzzle into a 100%-solved one. They also unlock medium puzzles, where naked singles run out earlier in the solve.

When to switch perspectives

The honest answer to "when do I look for naked singles versus hidden singles" is: scan for both, in passes, and switch when one stops yielding moves. A typical workflow on an easy puzzle:

1. Scan for naked singles cell by cell, place every one you see.
2. When the obvious naked singles run out, switch to the digit-first perspective. Pick the digit that already has the most placements (it's usually the easiest to find a hidden single for) and check each unit it's missing from.
3. After every hidden single you place, naked singles often re-appear elsewhere. Go back to step 1.

The ratio of moves usually goes seventy-thirty naked-to-hidden in the early stages of an easy puzzle, then flips toward hidden as the grid fills in.

The conceptual difference (and why it matters)

The naked single is a question about a cell: "what digit goes here?" The hidden single is a question about a digit: "where in this row does this digit go?" The same constraints apply in both cases — three groups, no repeats — but the question shape changes which patterns become visible.

This perspective-flipping is the foundation of every harder Sudoku technique. Naked pairs and triples are extensions of the cell-first perspective. Pointing pairs and box-line reductions are extensions of the digit-first perspective. The advanced techniques (X-wings, swordfish) usually require holding both perspectives at once. If the basic flip feels comfortable, the harder techniques are extensions of moves you already make.

If easy puzzles still feel like a struggle after you've internalised both singles, the issue is usually pencil marks rather than the techniques themselves. We've written about keeping pencil marks tidy without overdoing them as the natural next thing to learn. And once both singles are second nature, your easy puzzles start feeling more like crossword fill — small obvious moves in sequence, with the small satisfaction of watching the grid solve itself under your hands.