How to Spot an X-Wing Fast in Sudoku

How to spot an X-Wing fast

To spot an X-Wing fast, choose a digit and look for rows where that digit has exactly two possible cells. When you find two such rows and both confine the digit to the very same pair of columns, you have an X-Wing. The four corner cells form a rectangle, and the digit must sit at one diagonal or the other. Either way, both of those columns receive the digit from within these two rows, so the digit can be erased from every other cell in the two columns. The same works with the roles of rows and columns swapped, which doubles your chances of finding one.

Scan digits that appear often

The fastest route to an X-Wing is to scan digits that already appear about five or six times, since their remaining candidates are sparse and easy to compare. For your chosen digit, mark the rows where it has only two candidate cells; these two-spot rows are the building blocks. Then check whether any two of them line up on identical columns. Doing this digit by digit is far quicker than scanning cells, and it also warms you up for larger fish like the swordfish, which simply repeats the same idea across three rows and three columns. Building the two-spot habit pays off across many techniques.

A worked X-Wing

Take the digit 6. In row 2, 6 can only go at r2c3 and r2c7; in row 8, 6 can only go at r8c3 and r8c7. Both rows pin 6 to columns c3 and c7, so this is an X-Wing. Whichever diagonal is true, r2c3 with r8c7 or r2c7 with r8c3, columns c3 and c7 each receive their 6 from these two rows. That means 6 cannot appear elsewhere in c3 or c7. If r5c3 was showing 1 and 6 it becomes a bare 1, and any 6 in r4c7 or r9c7 is erased as well. One pattern clears several cells at once, which is why it feels so satisfying.

• Pick a digit that is already placed five or six times.
• Find rows where that digit has exactly two candidate cells.
• Check if two such rows share the same two columns.
• Erase the digit from the rest of those two columns.
• Repeat with columns and rows swapped to catch the other orientation.

Finding X-Wings in Shapedoku

An X-Wing is a matter of seeing one value repeat its position, and Shapedoku makes that easier by giving each value a bold color, so a shape confined to the same two columns in two rows almost lines up for you. Notes with smart cleanup trims dead candidates, which keeps those two-spot rows genuine rather than cluttered. If you are unsure a rectangle qualifies, the free solver on shapedoku.com confirms the X-Wing and lists what it removes. Start hunting them on Hard boards, then step up to Extreme mode at app.shapedoku.com, where a well-timed X-Wing often breaks a stubborn position wide open.