Only one cell in this box can hold that digit.
Technique 2 in the order the hint searches · difficultyMedium · up to hidden singles
Fix on one box and one digit. Follow the rows and columns where that digit already sits, ruling them across the box, and the places it could go shrink. When one cell is left, that's the answer.
Rule lines out from the digits already placed
First find a digit that already appears three or more times on the grid. The rows and columns holding it are your ruled lines, and a box that still lacks the digit will have very few cells left. The trick is to take in a whole horizontal band of three boxes at once.
This is how the hint puts it
In the top-centre box, 3 can only go in r3c5. Every other cell there is blocked.
Every box holds that digit exactly once
A 3 must appear somewhere in this box's nine cells. If only one cell can take it, that cell is the 3 — no matter how many other candidates are still written in it.
A puzzle that needs this much is labelled Medium. The logic is identical to scanning a line, but the movement your hand makes is different, so the two are kept apart.
Puzzles where this technique decides it
Four puzzles that won't give up the next move until you reach Hidden Single (Box). Press a number to open it.
- Practice 1 ·
1-9435761 - Practice 2 ·
1-8871522 - Practice 3 ·
1-7743044 - Practice 4 ·
1-7178805