Sudoku Sova

Sudoku technique · First needed at: Fiendish

Colouring

Follow the number's two-place chains, alternating as you go: one colour must be false, and anything seeing both colours is ruled out.

An example from a real puzzle

127946278927923582323532764724851289122958129346756379246746715798145984279516237235612798346725792472459279653471247923792347827983524679246792791249429137798237956
  • The cells that make the pattern
  • Candidates it removes

How to spot it

  1. Choose a number and follow its strong links: groups where it has exactly two places.
  2. Colour the linked cells in two alternating colours. One colour is true, the other false.
  3. A cell that sees both colours cannot hold the number.

Why it works

Along a chain of two-place links the number alternates, so one whole colour must be right — and a cell seeing both colours sees the number either way.

Sova teaches Colouring inside the game — and gives you puzzles to practise it on. Free on iPhone and iPad.

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