Medium Sudoku

The technique that unlocks medium

A hidden single turns the question around. Instead of asking "what can go in this square?", ask "where can this digit go in this row, column or box?" If a 4 is missing from a box and every square but one is blocked by a 4 elsewhere, that square must be the 4, even if other digits could also fit there.

Medium puzzles here start with 30 digits and are graded so that singles of both kinds are enough to finish them, without guessing.

Getting faster

• Sweep the grid digit by digit, 1 to 9, looking for boxes where that digit has only one home.
• After each placement, look at the row, column and box you just changed: new singles often appear right there.
• Keep notes light at this level. Filling every candidate is slow; note only the squares with two or three options.

Ready for more? Hard sudoku needs pairs and locked candidates.