Techniques / Basic Fish
X-Wing
A digit in two rows that is limited to the same two columns.
Take a digit. If in two different rows it has exactly two candidate cells each, and they sit in the same two columns, the four cells form a rectangle. The digit must occupy one diagonal of that rectangle, so it will be used once in each of the two columns.
Therefore, remove that digit from all other cells of those two columns. The same logic works with rows and columns swapped.
Tip: X-Wing is the smallest basic fish. Swordfish and Jellyfish extend it to three and four rows.
Notation
X-Wing: 6 can be removed from r5c7.
pattern removed
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