Medium Sudoku

Sudoku is played on a grid of 9 rows and 9 columns, split into nine 3×3 boxes. Some cells start with a digit (the clues). You fill in the rest.

Medium puzzles follow the same rules as every level; only the starting grid changes.

• Each row holds the digits 1 to 9, each exactly once.
• Each column holds the digits 1 to 9, each exactly once.
• Each 3×3 box holds the digits 1 to 9, each exactly once.
• The clues can't be changed. The puzzle is solved when every cell is filled and no digit repeats in a row, column or box.

A medium puzzle starts with 27 to 34 clues, most often about 30. Like easy, it never needs more than naked and hidden singles, but with fewer digits on the board there are fewer sure cells at any moment, and you will often have to check a digit across a whole row or column to find one.

Scan by digit. Go through 1 to 9 and, for each, check every box that is still missing it. This finds hidden singles that are hard to see when you look at one cell at a time.

Cross-hatch a band. The three boxes side by side form a band. When a digit is in two of them, it must be in the third box, on the one row the other two leave free. Often only one cell of that row is open.

Use notes for crowded areas. When a box has three or four empty cells, write the possible digits in each. A note that is the only one of its digit in a box is a hidden single; a cell with one note left is a naked single.

Tap or click a cell, then a number below the grid. On a keyboard, the arrow keys move between cells, 1 to 9 fill a cell, and Backspace or Delete clears it.

Notes turns the number buttons into pencil marks: small digits that remind you what can still go in a cell. Press N to switch on a keyboard, or hold Shift while you type a digit. When you place a digit, it is taken out of the notes in its row, column and box.

Undo takes back as many steps as you like. Hint first points out any wrong digit, then shows the next step a person could find, with the technique and why it works; press Hint again to fill it in. "Show mistakes" marks a wrong digit in red as soon as you enter it; switch it off to check your own work. Your game is saved in this browser, so you can close the page and come back.

Being stuck on a medium puzzle almost always means a single was missed, not that a harder technique is needed. Go back to scanning one digit at a time, starting with the digit that has the most copies on the board.

Ask for a hint if you want to keep moving: it names the row, column or box where the next single is, and the cell.