Strategy Detailed Descriptions Strategy Detailed Descriptions This is sometimes referred to as "Naked Candidates".
N-tuplets is a general term for couplets, triplets, etc. Couplets are most common, and triplets are also often found. Larger groups are hard to spot manually, but the computer will find them if they exist.
A couplet is when exactly 2 numbers are the only numbers Possible in 2 cells of a box, row, or column. Similarly, a triplet is when exactly 3 numbers are the only numbers Possible in 3 cells of a box, row, or column. To generalize, an N-tuplet is when exactly N numbers are the only numbers Possible in N cells of a box, row, or column.
Note that the N-tuplet numbers can also be possible in other cells of the box, row, or column; but no other numbers can be Possible in the N-tuplet cells.
When an N-tuplet is found, then those numbers are marked Impossible in all other cells of the box, row, or column that contains the N-tuplet.
In the row above, numbers have been marked impossible (details from other rows and boxes not shown). The first and fourth cells show only the numbers 5 and 6 as possible, so they form a couplet (2 cells that allow only 2 numbers). This means that the numbers 5 and 6 can be marked impossible in all of the other cells in the row.