Goal. Fill the grid so every row, column and box contains each digit from 1 to N exactly once.
Dots. Some cells have a dot between them and their neighbour. A hollow (open) dot means the two digits differ by exactly 1. A filled (solid) dot means one of the two digits is double the other.
Example: 3 • 5 with a hollow dot — the digits differ by 1. 4 • 8 with a filled dot — one digit is double the other.
Controls. Tap a cell to select it, then tap a digit. Toggle Notes to pencil candidates. Hint fills a correct cell. Undo reverts your last move.
Tiers. 4×4 Beginner → 6×6 Easy/Medium → 9×9 Hard/Expert/Master.