Lesson 1 of 9
Last free cell
One square left in a unit means the digit that unit is missing belongs in it.
What this grid does
- Starting digits
- 25 of 81
- Deductions to solve
- 56
- Uses of Last free cell
- 19
- Squares placed by scanning
- 56
- First needed at
- deduction 7, in row 7 (R7C1, R7C2, R7C3, R7C4, R7C5, R7C6, R7C7, R7C8 and R7C9, on the 4)
- Also calls for
- cross-hatching, naked single and hidden single
Measured on this grid by the same solver the hint button uses — how these puzzles are chosen.
Why this one is worth drilling
Every Sudoku needs this one — it is unavoidable, which makes it the technique you cannot avoid drilling either. That figure is measured rather than guessed: every logically solvable grid measured, at every clue count.
You cannot really be stuck on this one. If a unit has eight digits in it, the ninth is forced and no reasoning is required beyond noticing. The difficulty is attention, not deduction — which is exactly why it is first.
Where it sits in the nine
First because it needs no notes, no candidates and no vocabulary — only the willingness to count to nine. It is also the only technique here that stays useful at full speed right to the end of an expert grid: the last few squares of any solve are almost always last free cells, however hard the middle was.
Working it
Work this grid by unit rather than by square. Run your eye along each row, then each column, then each box, and count. The habit you are building is the sweep itself: every later technique starts by looking at a unit as a whole, and if you are still scanning square by square when you reach lesson five, the pairs will be invisible. One practical habit worth forming here: when you place a digit, look immediately at the three units it just completed a step of. A placement very often creates a last free cell somewhere adjacent, and catching those straight away is the difference between a two-minute easy grid and a six-minute one. Solvers who scan the whole board again after every entry are doing far more work than the puzzle asks.
The full explanation of last free cell covers the reasoning with diagrams.
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