From a pencil mark to a certain move.

A useful deduction tells you why a move must be true. Here is a small candidate example using the letters A–I. It is a unit illustration, not an entire playable Sudoku.

1. Write the missing letters

Suppose a row already contains D, E, F, G, H and I. Its three remaining cells must hold A, B and C.

2. Find the reservation

After checking columns and boxes, the first two cells each allow only A or B. Whichever one becomes A, the other becomes B. Those two letters are reserved for those two cells.

3. Use the consequence

The third cell cannot be A or B. Its remaining candidate is C, so C is a forced placement. You did not need to guess the order of the first two cells.

4. Check the condition

If either of the first two cells also allowed C, this naked-pair argument would fail. That is why complete, up-to-date candidates matter. A pair is a statement about all legal options, not just two letters you happened to pencil in.