- In the highlighted lines, 7 fits only in the highlighted cells.
- Whichever cells take 7, each crossing line gets one of them.
- So 7 can go nowhere else in the lines that cross them.
Swordfish in sudoku
A Swordfish is an X-Wing with three lines instead of two. Take one number and three rows where it can go only in cells of the same three columns. A row may use two of the columns or all three. The number can then be removed from the notes of every other cell of those three columns. Rows and columns can swap roles.
Or scan with your phone
How to spot it
Pick a number and list the rows where it is noted in two or three cells. Look for three rows whose cells, put together, fall into just three columns. A Swordfish rarely shows nine cells: six or seven is usual, which makes it harder to see than its name suggests.
Why it works
The three rows need the number three times, once each, and they have only three columns to do it in. So each of those columns gets the number from one of the three rows, and no other cell of the columns can hold it.
On a real puzzle
This is Swordfish on its practice puzzle, at the first step that needs it. The game's hint explains the step like this:
Practice it
This Extreme puzzle calls for Swordfish. Fill in the singles first. When no number is obvious any more, look for the pattern, or ask for the hint: each game has one.