Sudoku Techniques: Swordfish

As you continue learning more about the game of Sudoku, you’ll find that there is more to this simple yet addicting game than meets the eye. There are many terms and mechanics to learn and master, and there are more than a handful of useful techniques to choose from that can make solving a Sudoku puzzle a lot easier.

One of these techniques is called the Swordfish—a very useful tool to use when it comes to eliminating several number candidates at once.

Why is it named like that? Is it an easy Sudoku trick to learn? And how does it work in Sudoku puzzles? These answers will be answered as you continue reading more Sudoku tips and info below!

What Is the Swordfish?

The Swordfish is an intermediate Sudoku puzzle technique that is similar to the logic of how an X-Wing works— but involves more cells. There are no concrete records on why it’s called Swordfish and how it was created, but there are a lot of techniques in the Sudoku world that are named after marine animals for some reason. Some hardcore Sudoku players think it originated from the Sudoku programmer forums back in the 2000s, but there is no legitimate proof of this.

As with most Sudoku tricks, you need to fill in each row and column of the grid with enough candidate digits before identifying which cells you’ll use for the Swordfish. Once done, you can now look for a repeating candidate number among the cells. Let’s use this example:

You’ll be able to notice that we have the candidate number 4 repeating in cells r1c4, r1c8, r5c2, r5c4, r9c2 and r9c8. And there’s another aspect you’ll observe—the candidate 4 repeats in three columns and three rows. These are the most important parts of this technique. You’ll need to have a repeating candidate in three rows and columns. The Sudoku Swordfish is here.

What Can It Do in Free Sudoku Puzzles?

The Swordfish works as a beginning technique for medium Sudoku puzzles or higher difficulties as it can effectively eliminate a lot of candidate numbers in one go, given that certain conditions are met. In our example, there are six cells with the repeating candidate 4 and they fall within the three rows and three columns condition.

New players can find a lot of difficulty in understanding this technique, but we'll try to discuss it as clearly as we can. You can also take notes from other experienced players you interact with as well.

Using the Swordfish

Now, we begin using the technique. Let’s start looking at row 1, specifically, r1c8. We'll start using the logic of the technique from here and go end to end of the Swordfish.

If we start here and choose this as our first correct cell, then we remove the candidate digit 4 in Swordfish cell r9c8 as there can be no repetitions in the same column. Then we move to the next end of the Swordfish that was next to r9c8, which is r9c2. Since r9c8 is removed, then r9c2 must be correct. Then the nearest one is r5c2, which is eliminated since the previous one was deemed correct. After that, r5c4 is considered correct since the previous one is eliminated. And lastly, r1c4 is eliminated since the last candidate was chosen to be correct.

We are now left with three candidate 4s that are considered to be true, so by the rules of the game, we can now eliminate the other candidate 4s that are in the cells of each shared column.

Congratulations! You’ve done the Swordfish! Once you understand it more, you can significantly lower the difficulty of your Sudoku puzzles and fill in the grid with more confidence behind the numbers you press or write.

The Swordfish and Its Synergy With Other Sudoku Techniques

Since the Swordfish is a highly efficient candidate eliminator, it’s best used during the beginning stages of the game to remove as many numbers as possible. Think of it as using one of your more aggressive moves in an opening play.

However, keep in mind that it has to meet certain conditions to work, and not every puzzle can be effectively solved by it. If you're not confident in using it yet, try it out on easy Sudoku games right here on our site,

Further Sudoku Tips

Once you’ve used the Swordfish, it can pave the way for you to be able to use your other moves, such as the X- or Y-Wing, to whittle down even more of the pencil-marked number candidates. But remember, all these techniques are only as effective as the player that uses them. With proper usage, they can pull you out of a tight corner when you're having problems solving the puzzle.

They’re not cheat codes, but rather, tools that can help you solve the game more efficiently. Keep that in mind, choose your candidates wisely, take your time in playing and you’ll become a better Sudoku player in no time!