Hard sudoku

Why 25 clues change how you play

A hard sudoku here means 25 clues and a board that at least once requires a technique built on a group of cells rather than a single one. Scanning by eye will carry you to roughly half the grid and then stop. From there the game is about candidates — what a cell can still hold.

The practical consequence: on hard, pencil marks are a primary tool, not an emergency one. Without them you will not see the patterns this board rests on.

The techniques that are enough here

Our hard boards are calibrated to be solvable with this set — no guessing and nothing from the competition-solver toolbox:

• Locked candidates — if every spot for a digit inside a box sits in one row or column, that digit cannot appear in the rest of that row. And the other way round: if every spot for a digit in a row sits inside one box, it cannot appear elsewhere in that box.
• Naked pair — two cells in one unit hold exactly the same two options. Those two digits are reserved for those cells, so you can strike them from every other cell in that unit.
• Hidden pair — two digits that fit only in the same two cells of a unit. Those cells then hold nothing else, even if you had five candidates written in them.
• Naked triple — three cells that between them use exactly three digits. They do not each need all three; the union being three options is enough.

Nothing beyond that is needed. When a board looks like it demands something exotic, it almost always means a missed pair.

How not to lose three lives

The three-mistake limit is a real constraint on hard, so it is worth changing one habit:

1. Never write a digit you cannot justify. "It probably fits" is the most expensive move on the grid.
2. Put doubts in pencil. A mark costs nothing; a written digit costs a life.
3. Review your marks after every breakthrough in the row, column and box you touched. A stale mark is the single most common source of a mistake.
4. Conflict highlighting is off by default here because it gives away too much on this level. H turns it on, and nobody is judging.

The digit that cost you a life stays marked in red and struck through so you can see where to go back. Undo removes the mark but does not return the life.

"Very hard", "difficult", "tough" — is that another level?

Those labels mean different things on different sites, because nobody measures difficulty the same way. Here it is simpler: hard means 25 clues plus pairs and triples, and everything beyond that lives on expert sudoku — 21 clues and the hardest boards on the site.