Sudoku: number puzzles to solve here or print

How a Sudoku works

• The one rule: every row, every column and every box with a thick edge gets each number one time. In a 9 by 9 puzzle the numbers are 1 to 9 and a box is 3 by 3 squares.
• 4 by 4: the numbers 1 to 4, in boxes of 2 by 2. A first puzzle for a child, or for anyone who has not tried one.
• 6 by 6: the numbers 1 to 6, in boxes 3 squares wide and 2 squares tall.
• No adding: the numbers are only nine different marks. Nothing is added up or taken away.
• A way to start: look for a row, column or box that is nearly full, and ask which number is missing from it.

The three levels

• Easy: more numbers are given. The page stops taking numbers away when 40 are left in a 9 by 9 puzzle, 18 in a 6 by 6 and 8 in a 4 by 4.
• Medium: fewer are given: 32 in a 9 by 9, 14 in a 6 by 6 and 6 in a 4 by 4, in every puzzle the program checked.
• No guessing in those two: an easy or medium puzzle can be finished one sure step at a time. At each step either a square has only one number left that fits, or a number has only one square left in a row, a column or a box.
• Hard: the page takes away every number it can while there is still only one answer. In the 9 by 9 puzzles the program checked, 22 to 27 numbers were left. A hard puzzle may need more than the sure steps above, and some do not.
• The puzzle number: any whole number from 1 to 999999. The same number, size and level make the same puzzle on any device, so two people can work the same one. If we change how the page makes puzzles, a number will make a different one.

Every time the page is built, a program makes 1,500 puzzles (every size and level) and solves each one a second way, with a solver written apart from the page’s own. It checks that each has exactly one answer and that the answer on the page is that one, that each easy and medium puzzle is finished by the sure steps, and, for 440 of the hard puzzles, that taking away any one given number leaves more than one answer (3,861 numbers tried). It also measures the 24 ways the page can lay puzzles out on a sheet.

What this page does not do

• It does not keep your place. If you close or reload the page, the numbers you wrote are gone. The puzzle number brings back the puzzle, not your work.
• It has no timer, no score and no hints. Check my numbers tells you how many do not match and marks them; Show the answer fills it all in.
• It has no small pencil notes in the squares on the screen. On paper there is room for them.
• It does not grade the levels the way a puzzle book does. The levels are the plain rules above; one person’s medium is another’s hard.
• It makes no promise about your mind or your health. It is a puzzle to pass the time.
• It cannot see your printer. It has been tried in one browser’s print preview (Chromium, saved as PDF), where each sheet came out on one page; it has not been tried on paper from a real printer.