Naked Quads
Points. A
naked quad is formed by any 4 cells that have candidates for only 4 digits in
the same house. A naked quad is a naked subset of 4. Like Naked Triples, some
digits can be locked in more than one house, i.e., locked in a rowboxpair
or in a column box pair. What
Sudopedia Says.
Naked Quad, Row. In row
8, the four cells in columns 1, 4, 6, and 8 contain only the four digits,
2789 therefore the digits 2, 7, 8, and 9 cannot be present elsewhere in the
row, thus the red candidates 2 and 7 can be removed.


Naked Quad, Column. In
column 5, the four cells in rows 1, 5, 6, and 7 contain only the four
digits, 2, 7, 8, and 9. therefore, none of these digits can be present elsewhere
in the column, thus the red candidate 2 and 7 can be removed.


Naked Quad, Box. In box
5, the four cells marked blue contain only the four digits 1579 therefore
these digits 1, 5, 7, and 9 cannot be present elsewhere in the box. Thus
all the red candidates for 1, 5, 7, and 9 can be removed.


Naked Quad, Locked.
The last case is a special case where 4 digits, 1278, are locked in a
column, and, the digits 7 and 8 are also locked in a box, removing more
candidates. In this case the digit 8 in r6c8 can be removed in addition to
the digits 1,2 and 8 in clumn 9.


