Last Free Cell
Last Free Cell is the most basic move in Sudoku. When a row, column, or box has eight of its nine cells filled, the ninth cell has to take the one digit that's missing — there's nothing else to figure out. No guesswork, no candidates, just the rules of the grid doing the work.
What is Last Free Cell?
In a 9×9 Sudoku, every row, column, and box has to contain the digits 1–9 exactly once. Look at any of those 27 houses: if eight cells are already filled with distinct digits, the ninth cell — the last free cell — only has one value left. That value is whichever digit hasn't shown up in the house yet.
How to spot it
Scan rows, columns, and boxes for a sector with exactly eight filled cells. Compare the eight placed digits with 1–9; the missing digit belongs in the sole empty cell. A sector with only seven filled cells still has two empty cells and is not a Last Free Cell.
The logic
Every house has to hold each digit 1–9 exactly once. If eight are already in, the ninth has to be the one that's missing — there's no other candidate, because every other digit is already used in that house. That's the whole trick. It's the first technique every solver learns because there's nothing to argue about: the cell is empty, the digit is missing, and the rules force them together. The habit of scanning houses for near-completeness stays with you at every difficulty level.