Sudoku guide: from obvious cells to the X-Wing

1. Scanning: the only place for a number

The most used technique is to pick a number and a box, and rule out cells. To place the 5 in the top-left box, look at the rows and columns crossing it: every 5 already in those lines rules out a whole row or column of the box. Often a single free cell is left: that’s where the 5 goes.

This is a hidden single: the number has only one possible place in its row, column or box, even if that cell could still take other numbers. Easy and Normal puzzles are solved entirely this way.

2. The cell with only one option

The other view works the other way round: take a cell and see which numbers are already in its row, its column and its box. If together they cover eight different numbers, the ninth is the only one that fits.

This is a naked single. It shows up a lot towards the end, when rows are nearly full. Combining both views (by number and by cell) is what keeps you moving fast.

3. When to start taking notes

When neither scanning nor single cells give you anything new, it’s time for notes. In each empty cell of an area, write every number that could go there. You don’t have to do the whole grid at once: start with the box or row with the fewest free cells.

In Vexply, every correct number you place removes that number from the notes in its row, column and box, so your notes stay clean. Hold Shift while typing a number to note it without switching mode.

4. Locked candidates and pairs (Hard level)

Locked candidates: if inside a box the 3 can only go in cells of the same row, then that row’s 3 is in that box. You can remove 3 from the notes in the rest of the row, outside the box. It works the same with columns, and the other way round (a row whose 3 can only be inside one box).

Naked pairs: if two cells in the same row have exactly the same two notes, say 4 and 8, those two numbers are shared between them. No other cell in the row can hold a 4 or an 8. A hidden pair is the mirror image: two numbers that in a row only appear in the same two cells; those two cells can’t hold any other number.

• Look for pairs in the lines with the most notes: that’s where they remove the most.
• After every elimination, check for single cells again: they tend to appear right away.

5. Triples and X-Wing (Expert level)

A triple is a pair with three numbers: three cells in a line that between them only use three candidates (say 2, 5 and 9, even if one has only two of them). Those three numbers are shared among them and can be removed from the rest of the line.

An X-Wing looks at two rows at once. If in two rows the 6 can only go in the same two columns, the four spots form a rectangle and the 6s take two opposite corners. Whichever they are, each of those columns already has its 6 in those rows: remove 6 from the other cells of both columns.

The toughest Experts can call for reasoning by cases: “if this cell were a 1, that other one would have no options left”. Do it carefully, and use Undo if you try it out on the grid.

6. The daily Sudoku and your times

The daily puzzle changes at midnight and its level follows the week: Easy on Monday, rising through the week to Expert on Saturday. Solve it on consecutive days to build a streak; a day without solving it breaks the streak.

To cut your times, order matters more than finger speed: first a full scan number by number, then single cells, and only then notes. Turning on mistake marking saves minutes hunting for a wrong number, although every mistake is still counted.