Naked Singles
Short answer: a naked single is an empty cell where only one shape can go, because its row, column and box already hold the other eight. Find the cell, place the last shape, move on. It's the workhorse of easy solving.
What a naked single looks like
Take any empty cell and look at its three units — the row it sits in, the column it sits in, and the 3×3 box around it. If those units already contain eight different shapes between them, the cell has nowhere to hide: the one remaining shape is its only candidate.
Notice the deduction uses all three units. The row alone doesn't settle it, nor the column, nor the box — but together they eliminate every shape except one.
How to find naked singles
- Choose a promising empty cell. Naked singles hide in the busiest spots. Look at empty cells whose row, column and box are already well filled.
- List what its three units contain. Note every shape that already appears in that cell’s row, in its column, and in its 3×3 box.
- Count the distinct shapes. If those three units together contain eight different shapes, only the ninth can go in the cell — that is the naked single.
- Place it and rescan. Writing the shape in removes it as an option for every cell in the same row, column and box — which often exposes the next naked single.
A placed shape ripples outward: it becomes a new "already present" shape for every cell in its row, column and box, so naked singles tend to cascade. Solve one and immediately check its neighbours.
Naked single vs hidden single
These two are easy to confuse. A naked single asks "what can go in this cell?" and finds one answer. A hidden single asks "where can this shape go?" and finds one cell. Learn both — many puzzles yield to one when the other is stuck.
Frequently asked questions
What is a naked single in sudoku?
A naked single is an empty cell that has exactly one candidate left. Its row, column and box between them already contain eight of the nine shapes, so only one shape can legally fill it. It's the most basic solving move after the "last free cell".
What is the difference between a naked single and a hidden single?
A naked single is about a cell: the cell has only one possible shape. A hidden single is about a shape: the shape has only one possible cell in a unit, even if that cell could hold other shapes too. Both place one symbol; they just start from opposite ends.
How do I find naked singles faster?
Pencil in candidates for the empty cells, then scan for any cell showing a single mark. Failing that, focus on cells sitting where a busy row, a busy column and a busy box all cross — that overlap is where candidates run out first.