Sudoku (Hard)

Hard puzzles need the most advanced techniques on the site, and each of them still has a single solution you can reach by logic, with no guessing. Expect long stretches with nothing to place until you find the pattern that opens up the next part of the grid.

Triples extend the pair rule. When three cells in a unit can only hold, between them, the same three digits, those digits belong to those cells and come out of the rest of the unit. A hidden triple is three digits that only fit in the same three cells.

An X-Wing looks across two rows at once. If a digit can go in only two cells in each of two rows, and those cells line up in the same two columns, the digit must take one corner in each column, so you can remove it from every other cell in those columns. The same works with rows and columns swapped.

An XY-Wing uses three cells with two candidates each: a pivot holding X and Y that sees one cell with X and Z and another with Y and Z. Whichever digit the pivot turns out to be, one of the two wings must be Z, so any cell that sees both wings cannot be Z.

Keep your notes complete on Hard, because these patterns only show when every candidate is written in. If you get stuck, Undo is free and a hint shows you a next step. Puzzles here aren't ranked and are saved apart from the main page, so take as long as you need.