The game is very simple: there is a grid of 81 squares, divided into 9 blocks of 9 squares each. Some of these squares come with a written number and the goal is to fill the empty squares so that the numbers 1 through 9 appear only once in each horizontal row, vertical and within each of the nine blocks that form the grid.

Now we will see examples of each of these structures:

*Row*

A row is made up of 9 squares wide. The numbers 1 to 9 appear only 1 time in each of the squares of the same row.

*Column*

A column consists of 9 squares high. The numbers 1 to 9 appear only 1 time in each of the squares of the same column.

*Block*

A block consists of 9 squares, 3 high and 3 wide. The numbers 1 to 9 appear only 1 time in each of the squares of the same block.