Pointing Pairs
Pointing Pairs is an advanced strategy that relies on notes being properly filled in and helps you eliminate potential options for other cells. This strategy can be used when a number 1 to 9 is present in the notes of a 3x3 block twice, with both notes belonging to the same row or column. Since each number 1 to 9 must be used at least once per 3x3 block, this means that other notes within the same column or row but outside of the 3x3 block where the Pointing Pairs were found can be removed.
Pointing Triples
Pointing Triples is another advanced strategy that works like the Pointing Pairs. This strategy can be used when the notes within a 3x3 block’s row or column all contain the same number. Since the basic rules of Sudoku tell us that each 3x3 block must use the number 1 to 9 at least once, the row or column where these three notes appear must contain the number in the notes. This row or column can be extended outside the 3x3 block with any other cell containing the number in their notes to be removed.
X-Wing
X-Wing is an advanced Sudoku strategy which looks at two parallel rows or columns which contain the same number 1 to 9 in the notes. Once these rows or columns have been identified, the player needs to note which cells are diagonal from each other. Since the basic rules of Sudoku state that the same number can not be repeated twice within the same row or column, we know that these cells will be filled in diagonally, so they are in different rows or columns from each other. This allows us to remove this number from all the other notes in the columns or rows, except for the notes in the four cells that are diagonal from each other.
Y-Wing
The Y-Wing is an advanced strategy that uses the notes of three cells to eliminate notes in the fourth intersecting cell. This strategy starts by finding a cell with exactly two notes in it, which is referred to as the pivot. Next, we need to find two cells that are in the same row or column as the pivot which also contain two notes. These cells are referred to as pincers. Of the two numbers in the pincer’s notes, at least one needs to match one of the two numbers of the notes in the pivot cell. We can then extend the two pincers until they reach a cell where they intersect with each other. If the intersecting cell contains a note that is shared by both pincers, that note can be removed.
Swordfish
The Swordfish technique is an advanced Sudoku strategy that can be used by looking at the rows and columns where there are exactly two cells with the same number 1 to 9 as a note. If there are three columns and rows that use the same six notes with the same number from 1 to 9, the Swordfish strategy can be applied. Since these same six digits occupy the same three rows and columns in pairs, there are two possible combinations for the numbers to be placed. This means that all other notes containing this number outside of the selected six can be removed.