Expert sudoku

Every puzzle at this level is graded so that its hardest requirement is exactly the X-Wing pattern — a digit confined to the same two columns across two different rows (or the same two rows across two columns), which lets you eliminate it from the rest of those columns or rows. It's the first technique that asks you to track one digit across the whole board at once, rather than within a single row, column, or box.

X-Wing patterns are genuinely uncommon in a randomly-reduced grid — far less common than naked pairs or locked candidates — which is part of why this tier feels like a real step up rather than "hard, but more so." Pairs and triples alone won't move you forward here; you have to go looking for a digit that's cornered itself into exactly two rows and two columns.

How to actually find one

Pick a digit you haven't placed yet and check every row: does it have exactly two candidate cells left? If two different rows both answer yes, and both land in the same two columns, you've found it — clear that digit from every other cell in those two columns. The same logic works starting from columns instead of rows.

Who expert suits

Solvers who've stopped needing notes for pairs and triples and want the next real ceiling, not a harder label on the same logic.

Past X-Wing, there's evil — Swordfish, XY-Wing, and whatever a puzzle needs beyond the techniques most solvers ever learn.