Looking for a puzzle with Hidden Single…
Hidden Single
Solve one puzzle and let the hint show the technique at work. Unlimited hints, mistakes do not end the run.
Looking for a puzzle with Hidden Single…
A digit that has only one legal cell left in a unit, even though that cell still has other candidates.
Every row, column and box must contain each digit exactly once. So instead of asking what can go in this cell, ask where can this digit go in this unit. If the answer is exactly one cell, that digit belongs there — no matter how many other candidates that cell still carries.
It is called hidden because the winning candidate is buried among others. The cell does not look forced; the unit is what forces it.
Take one digit and one box. Look at the rows and columns that pass through the box and already contain that digit — those lines are blocked. Shade them mentally, and see how many empty cells in the box survive. One survivor means a hidden single.
Do this for a single digit across all nine boxes before switching digits. Digits that already appear six or seven times on the board are the most productive, because most of their lines are blocked already.
Most players cross-hatch boxes and stop there, which is why puzzles stall on Medium. The same counting works along a row: if a digit is missing from the row and every empty cell in that row except one is blocked by its own column or box, the placement is forced.
Hidden singles in rows and columns are the main engine of Medium puzzles on this site, where the solver is not allowed to use any elimination technique at all.
A naked single answers what fits here. A hidden single answers where does this go. They are the same deduction viewed from opposite ends, and a grid that has stalled for one often gives way instantly to the other. When you are stuck, switch the question rather than scanning harder.