Hard Sudoku

Hard Sudoku Puzzles

What makes hard sudoku puzzles hard?

There are several different rules for solving the numbers in particular cells. In rough order of difficulty from easiest to hardest these are the single cell rule - only one possible cell in a particular row, column, or 3 x 3 block where a particular number can go, single digit rule - only one particular number is possible in a given cell, various eliminations, x-wing, xy-wing, and swordfish. Most sudoku puzzles consist of many single cell rule steps. Hard sudoku puzzles have more solving steps at the difficult end of the range. The puzzles on this hard sudoku site are selected as having the most hard steps of all the sudoku puzzles we create.

Single Cell Rule

Only one possible cell in a particular row, column, or 3 x 3 box where a particular number can go.

Single Digit Rule

Only one possible number can go in a particular cell because all the other numbers already appear in the same row, column, or box as the given cell.

Pair Elimination

When two cells in a row, column, or 3 x 3 box both have the same two numbers as possibilities, then those two numbers can be eliminated as possibilities anywhere else in the same row, column, or 3 x 3 box.

X-Wing

Two pair eliminations that share one common number in matching rows, or matching columns, with the four cells forming a rectangle.

XY-Wing

Like X-Wing except one of the pairs is a single cell.

Swordfish

Like X-Wing but with more than two rows or columns or both. A subclass of this is Jellyfish with more than three rows or columns or both.

Printing

To print multiple puzzles, there are special pages to print two, four, or six puzzles per page.