Naked singles in sudoku
The naked single is the most honest move in sudoku: a cell that can hold exactly one digit. Its row, column and box between them already show eight of the nine, and whatever is missing goes in. Nothing to weigh, nothing to track. The skill is spotting these cells fast, because they hide in plain sight.
The pattern
Take the highlighted cell below and read what it sees. Its row supplies 1, 3 and 4. Its column adds 2, 8 and 9. Its box contributes 5 and 6. Eight digits accounted for, so the cell must take the 7:
The count is the whole technique. Selecting a cell on this site tints its row, column and box, which is exactly the set of digits to read off. Eight seen means place the ninth.
Where to look
Naked singles need crowded neighborhoods, so let the board’s busy regions point you. A box holding seven digits leaves its two open cells one constraint from resolving. A row with two gaps, a cell where a full-ish row crosses a full-ish column: these produce singles, and empty regions produce nothing. After every placement, glance at what that digit’s row, column and box just got closer to finishing; placements breed placements.
Near misses
One missing digit changes the answer. The same position with the box’s 6 removed:
Miscounting is the only way this technique goes wrong, and it goes wrong in one direction: you see eight where there are seven, place a digit the cell never earned, and poison everything after it. Count the digits, not your impression of them. With mistake checking on solution mode, the default here, a miscount turns red the moment it lands.
Where it leads
A two-candidate cell like the near miss above has its own name, a bivalue cell, and it stops being a consolation prize on higher tiers: bivalue cells are the fuel for naked pairs and the XY-wing. The naked single also has a mirror twin. When the cell stays open but the digit runs out of homes, you have a hidden single, and between the two of them plus scanning, every easy puzzle on this site falls start to finish.
Frequently asked questions
What is a naked single in sudoku?
A cell whose row, column and box together already show eight of the nine digits. One digit remains, and the cell must take it. The move needs no pencil marks and no lookahead, only the patience to count what the cell can see.
What is the difference between a naked single and a hidden single?
A naked single reasons from the cell: only one digit fits here. A hidden single reasons from the digit: only one cell in this unit fits it. The naked version is easier to spot and rarer; the hidden version powers most of an easy solve.
How do I find naked singles quickly?
Look where the board is crowded. A box holding seven or eight digits, a row with two gaps, a cell at the crossing of two busy lines. Empty regions produce nothing, so let the filled ones point you.
Why is it called naked?
The single stands exposed: pencil the candidates for that cell and exactly one appears, nothing hiding it. The naming carries through the ladder, where naked pairs and triples show their full candidate sets the same way.