Swordfish
A digit confined to three columns across three rows clears them.
The Swordfish sudoku technique is a single-digit elimination pattern — the three-row (or three-column) big brother of the X-Wing. It fires when one candidate is confined to the same three columns across three different rows, forming a 3×3 lattice that locks those columns down. Because each of the three rows must place that digit somewhere within the shared three columns, the columns are fully spoken for and the digit can be swept out of the cells in between.
Worked example
In rows 1, 5 and 7 the digit 6 has been squeezed into just three columns — 3, 4 and 7 — giving the candidate cells R1C3/R1C4/R1C7, R5C3/R5C4/R5C7 and R7C3/R7C4/R7C7. Since each of those three rows must put its single 6 into one of columns 3, 4 or 7, the three columns are collectively booked by rows 1, 5 and 7 and no other row may keep a 6 there. That lets you strike 6 from every other cell in columns 3, 4 and 7 — for example R2C3, R4C4 and R9C7.
When to use it
Reach for a Swordfish once singles, locked candidates and X-Wings have dried up but one stubborn digit still has too many candidates to place. Spot it by picking a single digit and listing which columns it can still occupy in each row: if three rows each restrict that digit to a subset of the same three columns, you have a base set. The clean case — like this board, where rows 1, 5 and 7 all show 6 in exactly columns 3, 4 and 7 — is the quickest to recognise.
How to use it
- Find three rows. Pick a digit and find three rows where it can only go inside the same three columns (each row uses two or three of them).
- See the lock. Across those three rows the digit must take three of the nine intersection cells — one per row and one per column — so the three columns are spoken for.
- Eliminate down the columns. Remove the digit from every other cell in those three columns. The same pattern works with rows and columns swapped.
Frequently asked questions
What is a Swordfish in Sudoku?
A Swordfish is a single-digit pattern spanning three rows (or three columns). When one candidate — here 6 — is restricted to at most the same three columns in three separate rows, those three columns can only be filled by those three rows, so the digit is eliminated from those columns in every other row. It is the natural extension of the X-Wing from two base lines to three.
How do I spot a Swordfish on the grid?
Focus on one digit and mark every cell where it can still go. Scan row by row for three rows in which that digit lives only within a shared group of three columns, each row using two or three of them. In this board, rows 1, 5 and 7 each pin 6 to columns 3, 4 and 7 — that shared trio of columns is the signature to look for.
Do all three base rows need the digit in all three columns?
No. A valid Swordfish only requires that the three base rows collectively use no more than three columns; any single row may hold the digit in just two of them. This board is the fully populated case — rows 1, 5 and 7 each carry 6 in all of columns 3, 4 and 7 — but a swordfish where some rows use only two columns eliminates just as reliably.