Swordfish
Short answer: find three rows in which one shape can only go inside the same three columns. Those three rows need three homes for the shape and only those three columns can supply them, so all three columns are spoken for — the shape leaves every other cell in them.
What a Swordfish looks like
A Swordfish is an X-Wing with one more row and one more column. That sounds like a small change and it is not: an X-Wing announces itself as a rectangle, and a Swordfish almost never looks like anything. Its nine potential corners are rarely all present, the rows involved need not be adjacent, and there is no shape on the grid to catch your eye.
Note what is not required: a row does not have to use all three columns. Two of the three is plenty. That is why counting corners is the wrong way to look for one — you are counting rows, and asking only whether each one stays inside the same three columns.
How to use a Swordfish
- Pick one shape and list its rows. Take a single shape and write down, for each row, which columns can still hold it. Rows with two or three possible columns are your candidates; ignore rows with four or more, and ignore every other shape while you do this.
- Look for three rows sharing three columns. Take those rows three at a time and pool their columns. If the pooled set has exactly three columns in it, you have a Swordfish. Four columns is a near miss and proves nothing.
- Check the rows are really confined. Every one of the three rows must have no home for the shape outside those three columns. One stray cell and the argument collapses — though that stray cell may make it a finned Swordfish instead.
- Strike the three columns. Remove the shape from every cell of all three columns that is not in one of the three rows. As with an X-Wing, the same pattern works with rows and columns swapped, so check both directions.
If a fourth column keeps sneaking in, try the pattern one size larger — that is a Jellyfish. If instead a single extra cell spoils an otherwise clean Swordfish, look at which box that cell is in: a finned Swordfish still eliminates there.
Swordfish vs X-Wing
The logic is identical and only the size changes, but the search is not. An X-Wing can be spotted visually, because two rows and two columns really do draw a rectangle. A Swordfish has to be looked for deliberately, one shape at a time, by writing down the columns each row allows. Solvers who never find Swordfish are usually not missing the pattern — they are scanning for a picture that a Swordfish does not make.
Find Swordfish in your own grid
Paste a puzzle and this page will look for Swordfish in it — the same pattern shown above, in your grid instead of ours. Nothing is uploaded; it runs in your browser.
Frequently asked questions
Does every row need three cells?
No, and this is the single most common misunderstanding. A row needs the shape to be confined within the three columns; two of the three is fine, and a Swordfish where every row uses only two is perfectly normal. What is not allowed is a row with a possible cell outside the three columns.
Do the rows have to be next to each other?
No. Rows 1, 2 and 6 work exactly as well as rows 1, 2 and 3, and so do the columns. There is no adjacency, band or stack requirement anywhere in the pattern — which is part of why Swordfish are so easy to walk past.
How often is a Swordfish actually needed?
Rarely, and less often than its reputation suggests. Most Swordfish you find will eliminate something an easier technique already reached, and many published "Swordfish required" puzzles fall to a skyscraper or a wing first. It earns its place because when it is genuinely the only move, nothing smaller will do.
Is a Swordfish the same as three pointing pairs?
No. Pointing pairs reason inside one box, and each one eliminates on its own. A Swordfish has no box in it: the three rows and three columns constrain each other, and no single row of the three proves anything by itself.