X-Wing
Prerequisites: Pointing Pair
Overview
When a candidate appears in exactly two cells in each of two rows, and those cells share the same two columns, the candidate can be eliminated from those columns.
How It Works
An X-Wing forms when a digit has exactly two candidate positions in each of two rows, and those positions line up in the same two columns. The four cells form a rectangle. Since the digit must occupy two diagonally opposite corners of this rectangle, it can be eliminated from all other cells in those two columns.
When To Use It
Look for X-Wings when you've exhausted simpler techniques. They are most effective when a digit has very few candidate positions in some rows or columns.
How To Detect It
For each digit, find rows where the digit has exactly two candidate positions. Then check if any pair of such rows shares the same two columns. If so, you have an X-Wing.
Visual Example
Step 1 of 11
Strong Links — A Digit With Only 2 Homes
Now focus on a single digit in a single row. In Row 1, digit 5 can only appear in two cells — R1C1 and R1C2 (both circled). Either one of them will eventually hold the 5 — we don't know which yet, but one of them must. This 'exactly 2 candidates in a house' relationship is called a strong link, or a conjugate pair.
Practice
An X-Wing on one digit is hiding here. Find it, eliminate, and fill the cell that drops to a single.