Hard Sudoku has usually X-wing or Y-wing, which are not hard to spot or solve at all.

Hard Sudoku puzzles can often be solved quite far (like in Sudoku Online game) before you realize that it's sort of hard. Here are the most common techniques to solve hard Sudokus.

If a candidate appears in two columns (rows) in exactly two identical rows (columns), it cannot be elsewhere in those rows (columns).

In this awesome X-wing example, two X-wings are eliminating the same candidate number at one cell. Can you solve this hard Sudoku?

X-wing is perhaps best understood by thinking of the corners that form the X-wing rectangle. Since the numbers of X-wing must be at the corners they must be opposite corners, otherwise the same number would be in one line twice.

The Y-Wing has three cells, which each has exactly two candidate numbers. The "center cell" shares one common candidate and block or line with each of the "tip cells". The tip cells also share a common candidate, which must always be in one of the tip cells, so it cannot be in cells that see both tip cells.

In the example, the red cells indicate a Y-wing. Which one candidate number does the Y-wing prune?

The Y-wing is perhaps easiest to spot by looking for a group of three cells with a total of three candidates. Y-wing is common and effective in eliminating candidates, especially when the other tip of the Y-wing is in the same block as the center cell.

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