Sudoku is a logic-based placement puzzle, also known as Number Place in the United States. The Sudoku board is made up of a 9x9 grid made up of 3x3 subgrids, for a total of 81 cells.
The goal is to fill in the empty cells, one number in each, so that each column, row, and region contains the numbers 1 through 9 exactly once. Each number in the solution occurs only once in each of three "directions," so to complete the puzzle, you need patience and logical ability.
We'll give you a few hints, or "given" numbers to get you started. There is only one unique solution to each Sudoku puzzle. People often scan left and right, or up and down to find and fill in the missing numbers, but don't forget that each region has one of the digits, too. When you've exhausted all the numbers you can find by scanning, then logic has to take over.
It's not a math puzzle, it's a logic puzzle. You could replace the numbers with letters, or any symbol of your choosing, and the puzzle would still be just as valid.
To start a new puzzle, click one of the four buttons at the top left depending on which difficulty level you prefer.
A fresh puzzle will be available each day for each of the four different skill levels. The first three levels - Easy, Medium and Hard - follow the tradition of creating puzzles with a symmetrical layout of starting, or fixed, numbers.
The fourth level -- Tough -- creates puzzles with as few fixed numbers as possible, sometimes as few as 22. At this level, the emphasis is on generating the most challenging puzzles that can be solved using logic, regardless of layout symmetry.
The pencil icon on the right of the board will place a number temporarily in a cell. Click the pencil, click the cell to highlight it, then click the number, and that number will be penciled in. The numbers are small so you could put several digits in each box if you wish. Some people use that technique to try possible solutions.
When you are sure of a number, click the pen icon, highlight a cell, then click the number to permanently place that number in the cell.
You can click "Check" to check your answers, or "Solve" to take away all the mystery, and give you the solution.
If you prefer to use the keyboard rather than the mouse, you may use the arrow keys to navigate around the grid. Use the number keys to enter a number in the highlighted cell. Hold the Alt key when entering a number to "pencil" it in. Pressing the same key again will remove the pencilled number. Press the space or Delete key to remove any numbers from the highlighted cell.
Although first seen in a US puzzle magazine in in 1979, Sudoku initially caught on in Japan in 1986 and attained international popularity in 2005.