How hard is Easy, Medium and Hard?

Easy

Every number can be found by looking at what's missing. Pick a row, a column or a 3 × 3 box, see which numbers it still needs, and check where each one can go. At Easy, there is always a square where only one number fits, or a number that fits in only one square.

• Good for: newcomers, relaxing solves, a quick puzzle with a cup of tea.
• Most people solve them without pencil marks, though they never hurt.

Medium

You also spot pairs and pointing patterns. Everything from Easy still works, but sometimes it runs dry. Then you look for things like:

• Pairs: two squares in a row that can only hold the same two numbers, say 3 and 8. Those two numbers must go in those two squares, so you can cross 3 and 8 off everywhere else in that row.
• Pointing: inside one box, a number can only go in squares that sit in the same row. Wherever it lands, it's in that row, so it can't go anywhere else in the row outside the box.

Pencil marks (small notes of which numbers could go in a square) help a lot from here on.

Hard

Chains of reasoning across the grid. On top of the Medium ideas, you'll sometimes follow a short chain: “if this square is a 4, then that one must be a 7, and then this one…”, and find that every way round leads to the same conclusion somewhere else on the grid.

Hard puzzles ask for patience and careful notes, but they stay fair: every step can be worked out, and they are still well short of the “expert” puzzles for specialists.