DukeJP2010 wrote:Thank you, I solved it. All I needed was to see that naked quad.

Any tips on how to spot these?

Flippant answer: Look for them.

What I mean by this is that for most people, naked quads will not just jump off the page at you. However, if you've put in pencilmarks and you look explicitly for them, it isn't so bad. Here are a few things to help you on your way.

1. To do an exhaustive search, look in each row, column and block.

2. To do a somewhat smarter search, look only at the rows, columns and blocks that have at least 5 unsolved cells (so in your puzzle, this already eliminates rows 1,3,4,6,8; columns 3,5,6,8; blocks 1,2,3,6 from having useful naked quads).

3. If a row, column or block has exactly 5 unsolved cells and it has a naked quad, then it must also have a hidden single. If you've already ruled out hidden singles in the grid, then you can rule out any row, column or block with exactly 5 unsolved cells. In your puzzle, this eliminates from contention rows 2,9; column 2; blocks 4,5,7,9.

Using 2&3, one only needs to check for naked quads (assuming you've already checked for hidden singles)

rows: 5,7 (there aren't any)

columns: 1,4,7,9 (there is one in column 9)

block: 8 (there aren't any)

4. No cell with more than 4 candidates can be a part of a naked quad.

5. If you aren't using pencilmarks, it is usually easier to spot hidden singles, hidden pairs, hidden triples and hidden quads. If you spot all of these first, the only time you'll miss a naked quad is if it occurs in a row, column or block with no solved cells.

I hope these help.