You've learnt about the Single Position and Single Candidate Sudoku solving techniques, you can solve easy and medium difficulty Sudoku puzzles with these techniques.
The next technique is the use of Pencil Marks. Pencil Marks themselves don't involve inserting values, but they make it easier to spot Single Candidate (in particular) and Single Position cells.
Pencil Marks are also required to start using the more advanced techniques.
Look at the highlighted cell,
There is a 3, 7 and 9 in that row.
There is a 5, 7 and 8 in that column.
There is a 5, 8 and 9 in that region.
This leaves the values 1, 2, 4 and 6 as possible values (or candidates) for this cell.
We will insert the values 1, 2, 4 and 6 in a smaller font size for this cell, these are called pencil marks.
Will now work methodically through each cell, and insert the pencil marks for each cell.
Filling in the pencil marks for all the cells gives us this grid.
This makes Single Candidates easy to spot, and makes Single Position easier to spot.
Having pencil marks makes Single Candidates very easy to spot - you are looking for cells that only have one pencil mark.
We can see 3 cells in this grid that can be filled using Single Candidates.
Single Position is easier to spot with pencil marks, but we have a little more work to do.
We can insert a 3 in the second row using Single Position - there is only one cell in the second row that has '3' as a pencil mark.
We can also insert a 3 in the 5th column - again, there is only one cell that has '3' as a pencil mark.
In summary, Pencil Marks don't allow us to insert any values directly, but they make Single Candidates and Single Position easier to spot.
Pencil Marks are also required to start using more advanced Sudoku solving techniques.
