NAKED QUAD

As a further extension of Naked Pairs and Naked Triples the highlighted cells in this case contain only the numbers 1,5,6 and 7. These numbers can therefore be removed from elsewhere in the central 3x3 sub-square. Fortunately, this leaves only a single 9 in C5-R6 which can immediately be solved.

Naked Quads can also appear in rows and columns, but are not encountered very frequently in published puzzles.