Sudoku strategy

Every Sudoku technique does one of two things: it places a digit, or it removes a candidate from a cell. Start with the simplest techniques and reach for harder ones only when the easy ones stop working. After every new digit, go back to the basics, because one placement often unlocks several more. New to the game? Start with the Sudoku rules.

Pick one digit and look at every box that is still missing it. The rows and columns that already contain that digit rule out cells in the box. When only one cell is left, place the digit there. Work through the digits 1 to 9 in turn; on easy puzzles, this alone gets you a long way.

A naked single is a cell that can hold only one digit. Look at everything in the cell’s row, column and box; if eight different digits appear across those three, the ninth must go in the cell. Naked singles are easiest to find where a crowded row crosses a crowded column.

A hidden single looks at the problem from the digit’s side: a digit that has only one possible cell in a row, column or box, even though that cell could hold other digits too. Say a row is missing 2, 4 and 9, and the columns block a 9 from every empty cell but one. That cell is the 9, even if a 2 or a 4 would also fit there on its own. Cross-hatching finds hidden singles in boxes; check rows and columns the same way.

When scanning stops producing digits, go cell by cell and note every digit that isn’t already in the cell’s row, column or box. It takes a few minutes, but it turns the rest of the puzzle into pattern spotting.

• Fill in the whole grid at once rather than a few cells at a time, so every later elimination has something to work on.
• Clean up after every placement. Remove the digit you placed from the notes of every cell in the same row, column and box. Online this happens automatically; on paper, do it by hand, since stale notes lead to wrong placements.
• Read your notes for singles. A cell with one candidate left is a naked single. A digit that appears in only one cell’s notes within a row, column or box is a hidden single.
• Use the keyboard online. Press N to switch Notes mode, the arrow keys to move between cells and 1 to 9 to add or remove candidates, all without touching the mouse.

If two cells in the same row, column or box have exactly the same two candidates, say both show only 3 and 8, then those two digits must go in those two cells, one each. Neither 3 nor 8 can go anywhere else in that unit, so you can remove them from every other cell’s notes there.

The same logic works with three cells. If three cells in a unit only hold candidates from the same three digits, for example 1 and 4, 4 and 7, and 1 and 7, those three digits are locked into those cells. That is a naked triple, and not every cell needs all three digits. A naked quad is the same idea with four cells and four digits, and it turns up far less often.

A hidden pair is two digits that appear in the notes of only two cells within a unit. If 4 and 6 can only go in two particular cells of a box, those cells must hold the 4 and the 6, so every other candidate in those two cells can be erased. The extra candidates make hidden pairs harder to see, so go through a unit digit by digit and count the cells each digit can use. Two digits that share the same two cells form a hidden pair.

These two techniques use the overlap where a box meets a row or a column.

X-Wing is usually the next technique players learn after pairs. Look for a digit that has exactly two possible cells in each of two rows, with both rows using the same two columns. Those four cells form a rectangle. The digit must sit in two opposite corners, and whichever pair of corners it is, each of the two columns gets one. So you can remove that digit from every other cell in both columns.

It also works with rows and columns swapped: two columns in which a digit has exactly two candidates each, lined up on the same two rows, let you clear that digit from the rest of those two rows.

On LetsPlayCards.Online the difficulty level sets how many clues you start with: 45 on Easy, 35 on Medium, 28 on Hard and 22 on Expert. It doesn’t pick puzzles by technique, so two puzzles at the same level can feel quite different. As a rough guide:

Easy

Scanning and singles usually finish the puzzle. Notes are optional.

Medium

Expect to write notes. Naked pairs and pointing pairs come in handy.

Hard and Expert

Full notes and every technique on this page. Every puzzle still has exactly one solution, so there is always a reason why a digit goes where it goes.

1. 1
   Recheck your notes. Make sure every placed digit has been removed from the notes in its row, column and box.
2. 2
   Scan all nine digits again. A digit you placed since your last pass may have created a new hidden single.
3. 3
   Look at nearly full units. Rows, columns and boxes with few empty cells are the easiest places to see what is missing.
4. 4
   Find cells with exactly two candidates. They are the building blocks of pairs, X-Wings and most advanced patterns.
5. 5
   As a last resort, test a candidate. Pick a cell with two candidates, try one and follow the consequences. If you reach a conflict, the other candidate is correct. Online, you can also select a cell and press Hint to get moving again; a hint never counts as a mistake.

Not updating notes

Remove a placed digit from every note in its row, column and box.

Writing notes too early

On easy puzzles, scanning is faster than filling in candidates.

Placing digits that “look right”

If you cannot say why a digit goes there, it is a guess.

Checking only boxes

Hidden singles hide in rows and columns too.

Leaning on the mistake counter

It tells you a digit is wrong, but not why. Find the logic instead.

Skipping the recheck

When stuck, rescan all nine digits before trying anything advanced.

• Keep a fixed routine: scan the digits 1 to 9, check crowded rows and columns, then write notes.
• Work where the news is. After placing a digit, look first at the row, column and box you just changed. That is where new singles appear.
• Stay on the keyboard. Arrow keys, digits, N for notes and Backspace to erase are quicker than clicking.
• Track your times. The timer stops when you finish, and your best time for each level is kept on this device, so you can see yourself getting faster.