
Finned X Wings 

A Finned XWing is a pattern that is one cell short of being an XWing  in other words, two columns that contain a candidate in only two rows, except for one (the fin), and vice versa. The logic behind this technique is that if the fin is not true, the rest of the XWing is.
Consider the example on this page. Columns A and F only have 7's in rows 2 and 4, except for the one in F5. If F5 is NOT a 7, then we have an XWing and can eliminate all other 7's in rows 2 and 4. However, if column F5 IS a 7, we can eliminate all other 7's in the middle Box. Either way, the 7 in E4 can be eliminated.
Not all fins must be only one cell. Sometimes a fin can extend to several cells that share the same tertiary unit. For instance, if in the example on this page the cell in F6 (which here contains a 2) was empty and contained the candidate 7, it would still be an extension of the same fin. The logic would then be as follows. If neither of the cells in the fin is true, the XWing is true. Otherwise, if either one of the fin cells is true, we can still make the elimination hightlighted in green.
Sudoku Snake gives Finned XWings a skill value of 90.






