Example of a Double
This is a screenshot
Two Squares in the same
that contain the same two
Possibles - and no others.
In this example, the Double is the two Squares containing the Possibles 7 and 8 (coloured green).
These two Squares MUST contain 7 or 8. Therefore 7 and 8 cannot be in any other Square in the same Row or Box. The 8
outlined in orange is an impossible solution and can be removed.