Learn Sudoku - Single Position

This article assumes you know the basics of Sudoku. If you need a quick introduction to the game, check out the Introduction to Sudoku.

All of the red words in this article are dynamic. Click on them to get helpful clues!

Single Position is the first technique used by nearly all beginners, and is certainly the easiest to start with. Being one of the fundamental scanning skills needed to complete a puzzle, the earlier you master this, the earlier you can move on to more advanced skills.

Before explaining what it is though, I have some good news and bad news. The good news is that no serious thought is needed to use this technique. The bad news is that this technique may seem a little tedious at first. Don't worry though! You will soon get much faster at it, and you will be able to use visual clues buried in the puzzle to help guide when to use it. But if you find yourself using mainly just this technique, chances are you will get bored. That's when you know it's time to move up to a more difficult puzzle!

The Single Position technique is the act of scanning a row, column, or box for only one possible empty cell that a given symbol can be placed into. For example, you may pick out a row in your puzzle and then check to see if there is only one empty cell that a 1 can be placed into. If not then scan the same row to see if there is only one place that a 2 can be placed into. Continue in this manner until you have exhausted all of the puzzle's symbols, then move onto the next row. Of course, this technique works the same for columns and blocks.

But if you are like me, a verbal description of this sort of thing will just put you to sleep. I need to see it in action! So let's go:

In this example 9x9 Sudoku puzzle, there are several opportunities to use the Single Position technique. To begin we'll look at the first column. Let's see if we can find a symbol that has only one possible location in this column. We'll start with the symbol 1. The top empty cell could possibly contain a 1 because there are no 1's in that column, row, or box. Looking at the next three empty cells we see that a 1 already exists in that block, and so it isn't possible that a 1 could go in any of those locations. And finally, for the last empty cell we see that a 1 also exists in that block, and so the last empty cell cannot be a 1. Click here to see a visual of which empty cells can possibly be the number 1 for the first column. Be sure to stare at this until you have figured out why a 1 can only go in the green cell.

From this we can deduce that the top empty cell in the first column must be 1. We can now fill it in and move on:

At this point you may choose to scan the same column for a Single Position for the symbol 2. You can do this now on your own if you like. If you do you will probably notice that there are a lot of empty cells in this column that a 2 could go in. Why? Because there currently aren't very many 2's in the puzzle to help narrow down the choices. But there are a lot of 1's, 3's, 6's, and 8's. A very good rule of thumb is to begin performing Single Position scans looking for symbols that are already filled in the most. Since we have already filled in a 1, let's stick with trying to find more 1's. To help out, click here to highlight all of the existing 1's in the puzzle. From this we can clearly see the 4 rows, columns, and blocks left that do not have a 1 filled in yet.

Let's continue our search for another Single Position for the number 1 by turning our attention to the bottom row. As noted earlier, there is already a 1 in the left-most block. Therefore the first two empty cells cannot be a 1. Likewise, the center block already has a 1 in it, and so the two middle empty cells cannot contain a 1. Now turn your attention to the last block. There are two empty cells in the bottom row in this block. There is already a 1 in the last column, which leaves only one possible location for a 1 in the last row. Click here to see a visual of which empty cells can possibly be the number 1 for the bottom row. And now click here to fill in the 1.

And that's it! You can keep going if you like. You can solve most of this puzzle using this technique.

Sometimes you may find yourself using this technique sporadically, and at other times you will methodically scan an entire puzzle. But as you get better at Sudoku puzzles, you will find that you will be able to multi-task; you will be able to perform this technique while also looking for other clues at the same time.

Just remember the following tip: Always start with the symbols that are already filled in the most. Also, it helps to get a bird's eye view of where those existing symbols are. This helps you to quickly find the areas that you need to focus on. If you can "highlight" in your mind all of a particular symbol in a puzzle (such as all of the 1's) you will find that your brain will start to pick up patterns. Trust me, stick with it and you will become lightning fast at this.

Now you're ready to learn about the Single Candidate technique!