Hard 16x16 Sudoku — Fish and Triples at Scale

Hard 16x16 sudoku is rated 3.0 to 4.9. It is the first level where scanning and simple pairs are not enough and you must find a pattern across several rows and columns. It suits players who finish medium grids and want fish and wings on a larger board. Expect a long solve.

What this level asks

Disciplined notes. A fish pattern on this grid can span several rows of sixteen cells each, so you need complete candidates in every unit you check. Working with notes from the start of this level is the main change from medium.

Techniques you will meet

• Naked and hidden triples (rated 3.6 and 4.0): three symbols confined to three cells of one house. With sixteen cells to choose from, triples hide well.
• X-Wing (3.2): a symbol limited to the same two columns in two rows, which clears it from the rest of those columns.
• Swordfish (3.8): the same idea across three rows and three columns.
• Y-Wing (4.2): three two-candidate cells linked in a chain that removes a shared candidate from cells that see both ends.

Triples and fish are the same patterns you may know from 9x9. The difference is the number of cells they sit among. A naked triple needs three cells whose combined candidates are only three symbols, and finding that among sixteen cells means reading every note in the unit. Fish patterns repeat the work across several lines, so a missing note in one row quietly breaks the whole pattern. That is why complete candidates matter from this level up.

Tips for this level

• Track one symbol through every row before moving to the next. Fish patterns show up this way.
• Fill full candidates only when singles and pairs have stopped.
• Use undo freely when testing a pattern.

Next step

If it feels too heavy, drop to medium 16x16. If swordfish and wings are routine, step up to expert 16x16.