Sudoku Techniques:

Hidden Pairs/Triples

Hopefully you’ve got the hang of finding Naked Pairs and Triples – if not, practise looking for those before trying to understand the hidden equivalent! Hidden pairs and triples are quite a bit trickier to spot – they’re hiding after all! This technique is also known as Hidden Subset or Unique Subset, in general.

Look in this highlighted area:

See that there are actually only two places where 1 and 3 can exist. You'd see them as two pairs, if one of them wasn't hidden by sneaking in an extra 2.

Using the same notation as before, just looking at the cells which haven't been fixed yet:

{46} {24} {13} {26} {123}

Because 1 and 3 can only exist in two of those cells (no other cells will accept either of them), that means they must be in those two cells, leaving no room for any other. Even though you don't know which is a 1 and which is a 3, you do know that the two isn't welcome, so you can remove it as a candidate from the end cell!

(The eagle-eyed readers will spot that you could have arrived at the same result by looking for the naked triple {46} {24} {26} - which would result in just the same - removing the 2 as a candidate from the end cell. This happens quite often!)

Looking for hidden groups

Just remember that you’re looking for a group of numbers that are limited to only a small group of cells. If you’re looking for hidden pairs, you’re looking for two numbers which only exist in two cells within that area – even though there will be other candidates in the same cell “hiding” them.

For triples, you’ll be looking for three cells, and so on.

Spot the hidden pair in this puzzle:

Even harder...

Look for the hidden triple for 3, 4 and 7 in this puzzle:

You should be able to remove the candidate for 1 from the top cell – but it’s definitely a challenge to spot!

Can you find the hidden triples in these lines?

The Elusive Hidden Quad?

Thankfully very few puzzles require finding a hidden quad to solve, because they're particularly tricky to spot, and devilishly hard to work out!

Even with the highlight to help you to know where to look, it might still take you a while to pick out the quad! A very unusual beast, is the hidden quad, not often seen in captivity!