A naked single is a cell that has exactly one candidate left. Its row, column, and box together already contain eight different digits, so only one value can legally sit there — the cell is “naked” because its last pencil mark is showing.
To spot one, look at crowded neighborhoods: cells whose row, column, and box are all busy. Keep pencil marks tidy and naked singles announce themselves — a lone small digit sitting in a cell.
It works by simple elimination. A cell shares a house with up to twenty other cells; every solved digit among them strikes one candidate. When eight distinct digits appear across those three houses, the ninth is the only survivor.
On the board
Look at the cell at row 2, column 4. Its row already holds 1, 8, 2, 4, and 6; column 4 supplies 7, 3, 9, and 8; and its box (the top-middle one) adds 2, 3, 4, 7, and 9. Between the three houses, every digit except 5 is spoken for — the cell’s pencil marks have collapsed to a single 5, and that is what R2C4 must be.
This is the exact board the app's interactive Naked Single demo uses — step 1 of its 55-step walkthrough, drawn here with its live pencil marks.
