Swordfish

The X-Wing idea with three rows and three columns.

A technique for Expert puzzles. Play a Expert puzzle

What it does

The X-Wing with three rows and three columns. Pick a digit: if in three rows it can only go in cells of the same three columns, those three rows place their copies of the digit in those columns, one in each. So the three columns get the digit from these rows.

How to spot it

Choose one digit and find the rows where it has only two or three places. Look for three such rows whose places all fall in the same three columns. A row may use only two of the three columns. The same works with three columns and three rows.

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Blue: where 3 can go in the base lines (grey). Pink: the lines that lose 3.These are the notes after the easier steps: some candidates are already removed, as they would be in your game.In rows 3, 4 and 9, 3 can only go in the blue cells, which line up in columns 2, 4 and 6. Whichever way it goes, columns 2, 4 and 6 get their 3 there, so it can’t go anywhere else in them.It removes 3 from row 1, column 2; 3 from row 1, column 4; 3 from row 1, column 6; 3 from row 2, column 2; 3 from row 2, column 4; 3 from row 7, column 6; 3 from row 8, column 2; 3 from row 8, column 6.

What it lets you do

You remove the digit from every other cell of those three columns (or rows, if you started from columns).

Try it

Find the Swordfish on this board. Pick the cells it clears, mark the candidates it removes, then check.

These are the notes after the easier steps: some candidates are already removed, as they would be in your game.

Practise it

The best way to learn a technique is to meet it in a real game. Practice puzzles are sure to need it, at Expert level; any Expert puzzle may need it too.

Practise with real puzzles Play a fresh Expert puzzle