## Goal

A Sudoku puzzle consists of a 9x9 grid, with a total of 81 cells. The goal is to fill the grid with the digits 1 to 9 in such a way that each row, column, and 3x3 section of the grid contains each of the digits. Several cells on the grid have already been given digits which can’t be changed. These cells are usually called givens, clues or fixed digits.

A well-formed Sudoku always have one solution, and the givens are placed in such a way that the solution always can be found using logical reasonining without having to resort to guessing.

## Pencil Marks

Many solving techniques requires the player to keep track of which digits can be placed in each cell. As it’s difficult to remember all these candidates a technique commonly used to keep track of them is to place pencilmarks (also known as hints) in each cell representing the possible candidates. These can later be used with various techniques that eliminates which candidates can be placed in each cell.