Medium sudoku

How medium differs from easy

There are 28 clues instead of 32, but the real change is elsewhere: on medium you reach a point where no empty cell has an obvious answer any more. You have to take an extra step — rule a digit out of where it cannot go, instead of looking for where it must go.

The most common shape of that is a digit trapped inside one row of a box. If every possible spot for a 5 in a 3×3 box sits in the same row, then a 5 cannot go anywhere else in that row outside the box. You write nothing down — and the board moves again.

Pencil marks stop being decoration

This is where marks start paying off, but only if you use them sparingly:

• Do not mark the whole grid. Writing every candidate in 50 cells takes longer than solving the puzzle.
• Mark one box or one row — the one you are stuck on. Once it breaks, clear the marks and move on.
• Mark pairs. Two cells in one unit holding the same two options are the most common breakthrough on this level.
• Marks clean up after themselves: entering a digit removes it from the marks across the whole row, column and box. You never do that by hand.

Toggle pencil mode with space or N; on touch, use the pencil button under the board.

The three mistakes that cost most here

• Entering a digit that is "very likely". That usually worked on easy. Here it costs a life, and you have three.
• Trusting old marks. A note you made ten moves ago may already be false if you have not looked at it since.
• Scanning rows only. On medium, 3×3 boxes produce more hits than rows and columns combined.

The cell where you lose a life stays marked in red and struck through, so you know what to fix — including when you have conflict highlighting switched off.

When to move on

You have medium covered when you can name the reason for every digit you write. That is the moment for hard sudoku: 25 clues, and boards where box elimination alone no longer gets you home.