Naked and hidden pairs

Hard

A pair is two cells in a row, column or box that will hold the same two digits, in one order or the other. You can’t tell yet which goes where, but you know those two digits are taken, and that’s enough to clear notes around them.

Naked pair

When two cells in a row, column or box can only be the same two digits, those two digits will fill those two cells. So no other cell there can have them.

272737372323232452345282682362626232457245723367343473636789573535785673692368924523453458348563699168543581492646597811791879354691819285142171A2B3C4D5E6F7G8H9I
In row F, F1 and F7 can only be 2 or 3. Those two digits fill those two cells, so the 2s in F4 and F5 go.

Hidden pair

The mirror image: when two digits fit in only the same two cells of a row, column or box, those cells must hold them. So any other notes in those two cells go. It’s called hidden because the cells’ other notes hide the pair until you look at the digits.

232325257127818917891223712713121212818235352313535167135167151681685825289895812121846975643495866945378796414298947327631483547691A2B3C4D5E6F7G8H9I
In row F, 6 and 7 fit only in F4 and F6. Those cells must hold them, so their other notes go: the 1s in F4 and F6.

How to spot them

  • Naked pairs show up in your notes as two cells with the same two digits, in the same row, column or box.
  • For hidden pairs, count the places each missing digit has in a unit. Two digits with the same two places are a pair.
  • The same idea works with three or four cells: triples and quads.

Try it on a real puzzle

Hard puzzles in Sudoku Knockout need techniques like this one, and never a guess. When you’re stuck, a hint shows you the step.

Play a Hard puzzle