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
Run your eye across the grid — row by row, column by column, box by box — and pause on the houses that are nearly done, the ones with seven or eight digits already in. Count what's there; whatever's missing goes in the last empty cell. Easy puzzles lean on this move heavily, but it never stops being useful. Even in a fiendish puzzle, the harder techniques eventually open up cells, and Last Free Cell finishes off the houses that get close.
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.