Duncan's SuDoku Solver

This is a version 13.0 screenshot

Buried Quadruple:

Four Squares in the same Row, Column, or Box that contain the same four Possibles - and the Possibles occur nowhere else in that same Row, Column, or Box.

In this example the Buried Quadruple is in the central Box and contains the possibles 1, 4, 6, and 9 (coloured violet). These four Squares MUST contain 1, 4, 6 and 9. Therefore they cannot contain any other Possibles. The 3, 5 and 8 outlined in orange are impossible solutions and can be removed.

NOTE: This analysis requires the Advanced Analyses license feature to be activated.