A Sudoku puzzle is composed of a 9 x 9 grid. The numbers already filled out in are called “givens”. Each number location is called a “cell”. In a 9 by 9 grid there are 81 cells. In this example the givens are colored in black.
In this puzzle there are 27 givens filled out. It is widely accepted 17 is the minimum number of givens a Sudoku puzzle can have and still be considered valid or solvable. There are no known valid Sudoku puzzles having 16 or less givens.
Next consider the following completed sudoku puzzle. The goal in solving the puzzle is to fill out the numbers 1 through 9 so that each number 1 through 9 only occurs once in each row, column, and 3 by 3 block.
The numbers we fill out as we solve the puzzle are called “values”. In this example the givens are in black and the values are in blue.
Take a closer look at each number in the solved puzzle. It is important to notice how each number occurs only once in each row, column, and 3 x 3 block. This only once idea is key in coming up with logic to decide which value we should pick for a cell when we are in the middle of solving a puzzle.