Sudoku: Play Sudoku

Sudoku, with a coach that explains every move. Most sudoku apps give you a grid and a hint button that fills in a square. This one tells you why: which technique applies, where it is on the board, and how the argument actually works. Then it gets out of the way and lets you find the next one yourself. ELEVEN DIFFICULTIES, PROPERLY GRADED From Very Easy to Beyond Hell. Difficulty is not guessed from how empty the grid looks — every puzzle is solved by the same ladder of techniques a person would use, and graded by what it genuinely requires. Most apps stop at roughly what a newspaper calls hard. That is the fourth tier here. HINTS THAT TEACH Hints arrive in stages. First the region worth looking at. Then the name of the technique. Then the whole explanation, drawn on the board: the cells that form the pattern, the candidates it rules out, and why it holds. Stop as soon as you have seen…