Medium Sudoku online
Medium stops solving itself with obvious moves alone: you'll need to pencil in candidates and combine more than one logical technique to keep advancing.
The logical challenge of Medium Sudoku
In a Medium Sudoku, sooner or later the obvious Naked Singles and Hidden Singles run out. To keep progressing without guessing, you need to pencil in candidates in the empty cells and use elimination techniques that rule out possibilities before you can safely place a number.
Techniques that come into play
Locked Candidates — when, inside a 3×3 box, every cell a number could go in falls in the same row or column, that number can be eliminated from the rest of that row or column outside the box.
Naked Pair — when two cells in the same row, column, or box share exactly the same two candidates, those two numbers can be eliminated from every other cell in that row, column, or box.
You can see detailed examples of both in Sudoku Techniques.
Before and after Medium
If Naked Singles and Hidden Singles still feel difficult, start with Easy Sudoku. If Medium already feels too easy, the next level is Hard Sudoku.