Naked and hidden pairs
When two cells in a house can only hold the same two digits, those digits belong to those two cells. No other cell in the house can have them.
Why it works
On the board above:
r5c2 and r5c8 can each only be 5 or 9.
So those two cells take the 5 and 9 for row 5. We don’t know which cell gets which yet.
That leaves no 5 or 9 for any other cell in row 5. Remove 9 from r5c9.
How to spot it
Look for cells with two candidates
Cells with just two pencil marks are the easy place to start.
Find a twin in the same house
Another cell in the same row, column or box with the same two digits.
Clear those digits nearby
Remove both digits from every other cell in that house.
For hidden pairs, count digits
In one house, find two digits that each fit in only the same two cells. Then clear the other marks from those cells.
Practice
Real boards from the solver, with every pencil mark filled in. Spot it, then use it, then find it cold.
Loading the boards…
Try it in a real puzzle
Each of these needs a pair or triple to finish. It opens in the player.
Tips
Triples don’t need all three digits in every cell
A cell in a triple can hold just two of the three digits. What matters is that the three cells hold only three digits between them.
Houses overlap
A pair in a row that also sits in one box clears the box as well.