This is an example of a completed Kropki Sudoku. Kropki Sudoku puzzles take place on a standard 9x9 Sudoku grid, but there is a mixture of black and white dots between the cells to tell you about the relationship between the values in those cells,

• Black dots: One of these cells is double the value of the other cell.
• White dots: One of these cells is one bigger than the other cell, i.e. they are consecutive values

It's also important to note that if there isn't a dot between two cells, that means that they are neither consecutive or double the other, i.e. all dots are given.

This is a starting Kropki Sudoku grid of medium difficulty. I have highlighted 2 cells that we can fill in to start with.

The highlighted cell in the top-left is consecutive with the 8, i.e. the highlighted cell must be a 7 or a 9. The normal rules of Sudoku also apply and we can see that we already have a 7 in this column, this means the highlighted cell must be a 9.

The highlighted cell at the bottom of the grid has two relationships we can look at,

• It is consecutive with 5, i.e. it must be 4 or 6.
• It is double or half of 3, i.e. it must be 1.5 or 6.

We can't insert decimals in to a Sudoku grid, so this clue relation alone tells us the highlighted cell must be 6. This is allowed by the first relationship (consecutive with 5), and that is always good!