Kept in this browser only. Nothing is sent anywhere.
Keys: digits to place, arrows to move, P pencil, H hint, Z undo, Y redo, Delete to erase.
Every row, column and box holds each digit once. Each puzzle is made on your device from a seed, then cells are taken away one at a time as long as only one solution is left, so it never needs a guess to be fair.
Diagonal adds the two long diagonals. Knight stops equal digits a knight’s move apart. Irregular swaps the boxes for jigsaw regions. Odd and Even shades cells that must be one or the other. Killer adds cages with a sum, and Thermo adds thermometers that rise from the bulb.
A hint finds the simplest technique that makes progress, names it and shows where, in plain words. There are nine, from a naked single to an X-Wing, and the level of a puzzle is the hardest ones it needs.
Today’s puzzle is made from the date, so everyone gets the same one, harder later in the week. A link holds a whole puzzle, and you can type in one from a newspaper to have it checked, graded and played here.
A 9 × 9 puzzle with 16 clues cannot have exactly one solution. McGuire, Tugemann and Civario showed it in 2012 by testing every possibility, so 17 is the smallest number of clues that can work.
There are exactly 288 ways to fill a 4 × 4 grid. This page’s own check counts them by search, which is a good test of the rules before any puzzle is made.
There are 6,670,903,752,021,072,936,960 completed 9 × 9 grids (Felgenhauer and Jarvis, 2005). Treat grids that are just relabelled, turned, reflected or shuffled as the same and only 5,472,730,538 are left.
Every row, column and box adds to 45. In a Killer puzzle that is how a cage tells you what a neighbouring cell must be: a group of cages that fills a whole row leaves nothing to chance.
Naked and hidden singles, locked candidates, naked and hidden pairs, naked triples, X-Wing, cage sums and thermometers. A puzzle that needs more than these is graded Expert.
Every variant is the same idea: groups of cells that must hold different digits, plus extra rules on some cells. A single solver checks that each puzzle has one answer, whatever the rules.
A level is this page’s own measure of the techniques a puzzle needs, not a published rating, so it will not match another site’s Hard. Small grids are always easy to solve by logic, so their level only sets how few clues you get, and 16 × 16 has no Expert.
Yes. Cells are removed one at a time, and a removal is kept only when a search over every possible fill still finds exactly one answer. A puzzle you type in is checked the same way, and you are told if it has none or several.
The puzzle is solved the way a person would, with the simplest technique that works at each step, and the techniques used are counted. Easy needs only singles. Medium needs a few locked candidates or pairs. Hard needs many of those, or a triple or an X-Wing. Expert needs three or more of the hardest kind, or more than the coach knows.
When no technique it knows applies, the puzzle needs something harder or a guess. It then tells you so, and can show one cell from the solution instead.
It is made from the calendar date, so everyone who opens it that day gets the same puzzle, easy at the start of the week and harder towards the end. Your streak counts the days in a row you finish it, on this device.
An unfinished puzzle, your best times and your daily streak, in this browser only, so you can carry on after closing the page. Nothing is sent anywhere, and clearing the site’s data in your browser removes it.
Classic, Diagonal, Knight, Irregular, Odd and Even, Killer and Thermo on 9 × 9, and the classic rules on 4 × 4, 6 × 6 and 16 × 16 grids.
No change to this page at all since the earliest archived release (v8.31). The full history for the site is in the changelog.