Polka (Mind Loft)

Polka is the kropki sudoku on Mind Loft, a free site of daily browser games. The grid follows normal sudoku rules, with each row, column and 3x3 box holding 1 to 9 once, but not one digit is printed at the start. All the information is in the dots on the borders between neighbouring squares.

A white dot means the two digits differ by exactly 1. A black dot means one digit is double the other. No dot at all means neither is true, and that is what makes this version hard: every blank border rules out pairs, and those silences do much of the work. Each day's grid has a single solution that can be reached by logic without guessing.

The game does not flag wrong entries, so you need to check your own work. A clean solve scores 10 and the leaderboard is ordered by time. The usual helpers are there: notes, auto-move, undo, erase, clear and a hint, plus a count of each digit still to place.

A new grid goes up at midnight US Eastern Time. The boards get harder as the week goes on, and the Sunday Edition is drawn from the top of the difficulty range. Polka sits in Mind Loft's sudoku family with Suds (classic), Mercury (thermo) and Cages (killer, which also opens empty).