

SwordFish Strategy
With XWings we looked at a rectangle formed by four numbers at the corners. This allowed us to exclude other occurrences of that number in either the row or column. We can extend this pattern to nine cells and achieve even more eliminations.
A SwordFish is a 3 by 3 ninecell pattern where a candidate is found on three different rows (or three columns) and they line up in the opposite direction. Eventually we will fix three candidates somewhere in those cells which excludes all other candidates in those units.
The shaded cells show the SwordFish where X is unique to three cells in columns 2, 4 and 6. They are aligned on rows A, C and F. This means we can remove all candidate X in the other positions on those rows.


If you are not convinced that the shaded cells really must contain the solutions we can argue this way. All SwordFishes will break down into XWings and because we know XWings work, so will the SwordFish.
Take this arrangement of candidate A and let’s pretend that E6 is the solution. We ‘remove’ the rest of A in column 6 and row E. That leaves a XWing in AC24.

XWing inside a SwordFish 
If that works for E6, let’s try another cell. Pretending C2 is a solution we remove the rest of A in row C and column 2. Again we get an XWing. So all cells in the 3 by 3 grid are ‘locked’ together.

Another way to cut it 
To match theory with practise the first example is a perfect 333 Swordfish, so called because all three candidates in each column are present (that is, no solved 8s in the pattern). The yellow cells are the Swordfish cells. The green cells are those cells where 8 can be removed.
A perfect SwordFish is extremely rare. This one is provided by Klaus Brenner who found it in the newspaper La Libre Belgique.

Perfect 333 Swordfish: Load Example or : From the Start 
If you remember how Naked and Hidden Triples work you'll remember that they require three numbers in three cells  in total. It's not necessary for every number to be in all three cells. So it is with the SwordFish.
Swordfishes come in a number of variations depending on the number of X present in the nine cells that make up a Swordfish. With an XWing you need candidate X in all four cells of the 2 by 2 formation, but with the 3 by 3 Swordfish formation you don't need X in every cell  just as long as it is spread out over 3 by 3 cells. The next example has 9 twice in each column and is called a 222 Swordfish.
This is a 222 formation SwordFish in the columns and eliminates in the rows. I have labelled the three pairs AA, BB and CC which form each "2" in the name. Notice how they are staggered so that they still cover three columns. This is a minimal SwordFish but it does the job. We have six 9s that can go in one swoop.

Swordfish Example 1: Load Example or : From the Start 
This second SwordFish is orientated in the opposite direction and we eliminate in the columns.
A SwordFish can be referred to by combining the row and columns numbers, which makes this example CDJ379. In formation terms it is 323.

SwordFish example 2: Load Example or : From the Start 


