Sudoku

Master sudoku

Master sudoku is a step up from Expert. Beyond the X-Wing and XY-Wing, a puzzle at this level can ask for a Swordfish, an XYZ-Wing, a W-Wing or simple colouring, where you follow one number through a chain of cells in which it must be either here or there.

Cell colouring helps a lot: paint the two sides of a chain in two colours and the contradiction becomes visible. Every puzzle still has one solution and never needs a guess.