Sudoku Basics: Candidates & Pencil Marking

The basics of Sudoku

Solving a Sudoku puzzle doesn’t have to be difficult. Once you know the rules needed to play, common terms in the game, and the basics of scanning, placement and elimination, you can then start learning more to go beyond the limitations of these starting skills. Pencil marking and candidate placement are very essential skills to have in helping you get the results you need to solve your easy Sudoku puzzle.

These two Sudoku skills are also the groundwork you'll need to learn more advanced techniques to start solving medium Sudoku puzzles and play harder grids in general.

But what is pencil marking? And why do candidates matter in the game? Read down below to find out and learn more Sudoku tips!

What Are Candidates?

Candidates are temporary numbers or digits that a Sudoku player places in a cell to fill them in with possible correct digits. Depending on your decisions, you can place small numbers of 1 to 9. Let's use this sample Sudoku grid to make it easier to explain.

Now, let’s focus on the topmost boxes, boxes 1, 2 and 3. We’re going to start solving these boxes by filling in the empty cells, and we’ll start scanning from number 1. From that, you can observe that only box 1 has the number 1, and the other two don’t.

To begin, take a look at box number 2. Where should the digit 1 be placed here? We can’t put it anywhere in row 3 since there’s already a 1 there in box 1. So the logical answer is that we can place it in either row 1 or 2. However, we can’t put it a 1 on r2c5 since there is already a 1 in column 5, so the 1 could either be in r1c4 and r2c6!

But we can’t put two 1s in this box, as Sudoku rules prevent that. So how do we make it easy to remember that these two cells could contain the right number 1?

That’s when we use pencil marking to make the game easier.

What Is Pencil Marking?

Pencil marking, also called notations by other Sudoku players and game enthusiasts, is a process of placing temporary digits into a cell to help make the process of solving easier. After all, there are 81 cells in the grid, and it is very difficult to remember each possible correct digit for each cell as you continuously solve. Pencil marking works as the temporary number marker you need as you check each row, box and column.

The original classic Sudoku game was created on a sheet of paper, and it’s difficult to finish it using permanent writing instruments like pens and ink markers. This technique is named “pencil marking” because players wrote candidate numbers down using pencils as they could be erased and replaced as many times as needed. An average online Sudoku site features different ways to pencil mark, like pressing on a number pad or entering the numbers yourself in a type box.

Regardless, these notes are extremely essential in efficiently solving a Sudoku puzzle.

How Does It Work?

Pencil marking is essential as it reduces the difficulty of eliminating numbers and organizes your logic process in general. Imagine having to mentally take note of nine possible numbers for 81 cells. Almost impossible, right? That's why most modern iterations of Sudoku apps have a candidate and pencil marking feature.

Now, back to our example. We deduced from our scanning that the 1 for box 2 could either be in r1c4 and r2c6, but we don’t know which. We now pencil mark these two cells with a number 1 to make them easier to remember and get back to. The size and placement of your orientations are all up to you, but the most standard way of pencil marking cells is like this:

1 2 3
4 5 6
7 8 9

This is to make it more uniform to the eye and make your candidates easier to recognize even in your peripheral vision.

Solving Free Sudoku Puzzles Using Both Skills

We now continue playing and pencil mark both r1c4 and r2c6 with candidate 1. With this, our goal is now to prove which one of these candidates is the correct one. Since we can’t confirm which of these is correct in box 2, we’ll move to box 3, which is a lot emptier compared to the other two.

This box doesn’t have a 1, so let’s find the proper placement. Using scanning and intersections, we determine that we can’t place 1 in row 3, column 8 and column 9. This leaves us with one possible answer: cell r2c7. Now that box 3 has a 1, we can now confirm which of the candidates in box 2 is the correct one, and since the 1 in box 3 is in row 2, we can eliminate candidate 1 in this whole row.

We can now confirm that r1c4’s candidate 1 is the correct one, and we can now “pen” it in with a real number 1.

Congratulations! You’ve just learned how to use candidates and pencil marking in Sudoku! You can continue practicing these two skills, along with the others you've picked up, on easy Sudoku puzzles first. The world of Sudoku is vast and exciting, and you don't need to hurry yourself in learning more advanced techniques. Determining candidates and making notes is just the start, and you can pace your learning experience to your comfort and preference.