Every sudoku can be solved by logic alone — no guessing is needed. Most easy and medium puzzles need only two techniques, singles and pencil marks; harder puzzles add pairs and pointing.
Hidden singles
Pick a number and a box. If there is only one cell in that box where the number can still go — because the other cells are blocked by the same number in their row or column — that cell must hold it. Repeat for every number and every box.
Naked singles
Look at one empty cell and list which numbers are still possible. If only one number is left after checking its row, column and box, that is the answer for the cell.
Pencil marks
On harder grids, write all candidates in each cell. Every time you place a number, remove it from the notes of the same row, column and box. Patterns that are invisible in a plain grid become obvious with notes.
Naked pairs
If two cells in the same row, column or box both contain exactly the same two candidates, those two numbers must go in those two cells. You can remove them from every other cell in that unit.
Pointing
If, inside one box, a candidate only appears in a single row or column, the number must be in that line within the box. You can remove that candidate from the rest of the line outside the box.
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