A sudoku is a grid with 9 rows and 9 columns. There will be some numbers specified within the cells of the grid. The specified numbers given at the start of the sudoku are called “givens”.

The smallest unit within a sudoku is a box called a *“cell”*, that may be either initially empty or contain a number (a *“given”*). A sudoku grid contains \(9 x 9 = 81\) of these cells.

In order to solve a sudoku, you need to place the numbers from 1 to 9 within each cell, so that each row contains a unique number, each column contains a unique number, and each block contains a unique number. This means that you can’t have the same two numbers in a row, column or block. The givens restrict the numbers that can be placed in the empty cells.

Cells may be identified by their row and column. The cell at the top-left corner is labelled “1,1”, which stands for the location; row 1, column 1. The bottom-right-most cell is labelled “9,9”. The cell at the very centre is “5,5”.

Within the main sudoku grid are 9 *“blocks”* or groups of 9 cells composed of 3 rows and 3 columns. Blocks are outlined in thicker lines than individual cells. These are named “Block 1” starting at the top left, “Block 2” at the center-top, through to “Block 9” at the bottom-right.

Every cell has a set of “related” cells. They are the cells in it’s row, column or block. If you put the number “3” in cell, then the number “3” cannot be placed in any other related cell.

A properly formed sudoku has a unique solution - that is, you can only place the numbers in one way that guarantees a unique number in each row, column and block. Solving the sudoku means determining what this unique solution is.