Sudoku Strategies: 10 Techniques from Beginner to Expert

Every Sudoku can be solved with a handful of patterns. Easy puzzles need only the first two or three; expert puzzles ask for all of them. This guide covers 10 strategies in the order you’ll need them, each shown on a real position from a real puzzle. The highlighted cells, removed candidates and placed numbers in every diagram come straight from our solver, the same one that powers the hints in the Sudoku.my app.

New to the game? Start with How to Play Sudoku, then come back here.

All 10 strategies at a glance

1. Hidden single

The workhorse of Sudoku. Pick a box and a number it still needs. Every copy of that number in the rows and columns crossing the box rules out cells. When only one cell is left, the number goes there, even if that cell could hold other numbers too. That’s why it is “hidden”.

How to spot it: go number by number (all the 1s, then all the 2s…) and visit every box. Numbers that are already placed many times are the quickest to finish.

2. Hidden single in a row or column

Exactly the same idea, applied to a row or a column. These are a little harder to see because the cells are spread out, so many players check boxes first and lines second.

3. Naked single

Look at one cell and count what its 20 peers (the other cells in its row, column and box) already use. If eight different numbers are taken, the ninth is the answer. Naked singles become common later in a puzzle, when cells are surrounded by filled peers.

4. Pencil marks (candidates)

When singles run out, write the candidates for each empty cell: every number not yet used by its row, column or box. Every strategy below works on these notes. Keep them accurate. After placing a number, remove it from the notes of all its peers (Sudoku.my can do this automatically in Settings).

5. Locked candidates

If, inside a box, every possible spot for a number lies on one row or one column, that number must be in that line within the box, so you can remove it from the rest of the line. This is called pointing. The reverse, claiming, also works: if in a row or column a number can only go in cells of one box, remove it from the rest of that box.

6. Naked pairs and triples

If two cells in the same unit can only hold the same two numbers, those numbers must go in those two cells. They are “used up”, so remove them from every other cell in the unit. The same works with three cells sharing three numbers, a naked triple. A triple’s cells don’t each need all three numbers: {3,4}, {4,5} and {3,5} still form a triple.

7. Hidden pairs and triples

Now look at numbers instead of cells. If two numbers can only go in the same two cells of a unit, those cells must hold those two numbers, so any other candidates in them can be erased. Hidden pairs are easy to miss because the cells are cluttered with other notes.

8. X-Wing

Pick one number. If in two columns that number can only go in the same two rows, the four cells form a rectangle. The number must sit in two opposite corners, and either way it fills those two rows. So you can remove it from every other cell in those two rows. (Swap “rows” and “columns” for the mirror version.)

9. Swordfish

A Swordfish is an X-Wing with three lines. If in three rows a number can only go in the same three columns (each row using two or three of them), the number fills those three columns from these rows. Remove it from the rest of the three columns.

10. XY-Wing

You need three cells with exactly two candidates each: a pivot with {X,Y} and two wings it can see, with {X,Z} and {Y,Z}. Whatever the pivot turns out to be, one of the wings must be Z. So any cell that sees both wings can’t be Z.

A 4-week practice plan

Learning techniques in order, one at a time, is the fastest way to improve. Here is a plan that takes about 15 minutes a day:

Mastered all ten? The same techniques power the popular Sudoku variants. A good next challenge is Killer Sudoku, which adds cage sums to everything you’ve learned here.

#Techniques#X-Wing#Advanced