Sudoku

Sudoku on Greatest Games is a free daily logic puzzle from a San Francisco studio that publishes a new grid every day at midnight US Eastern, playable in the browser with no account and nothing to download. Each day brings four different puzzles, Easy, Medium, Hard and Very Hard, each timed independently, and you choose your difficulty before you solve. The site calibrates the tiers in its own notes: the Easy board carries 62 given digits and only 19 blanks, so singles alone usually carry you to the end, while Very Hard starts from 35 givens where singles are only the opening act. The rule is the classic one, every row, column and 3 by 3 box filled with the digits 1 to 9 without repeats, and below the board the site publishes a scoring panel, a FAQ and a long note on the origin of the puzzle and on solving technique, which makes the page as much an article as a grid.

1. Choose your difficulty, Easy, Medium, Hard or Very Hard, and change it from the Settings panel if you want another; the LEVEL and LEFT counters above the grid show the tier and the number of empty cells.
2. Tap a cell, then tap a digit on the pad or type 1 to 9. Tapping the same digit again clears the cell, and the Erase button empties it.
3. Switch on Notes, the pencil button or the N key, to pencil small candidate digits into a cell while you reason; notes do not count as entries.
4. Read the colours: given digits are printed plainly, your entries appear in the accent colour, and a digit that clashes with a matching one in its row, column or box turns red.
5. Fill every row, column and 3 by 3 box with the digits 1 to 9 without repeats, and clear every red cell, because reds must be gone for the grid to count as solved.
6. Check the scoring panel once you finish: 100 points for the solve, one point per day of your current streak, and one speed bonus if you beat the tier's time cutoff.

Because the difficulty choice is spelled out rather than decorative: you know how many digits you get before you click, and the site's notes tell you which technique cracks each tier, from cross-hatching on Easy to box-line reduction on Hard. It is a sudoku for people who also like reading about sudoku, and for players who enjoy a points race, every solve scoring 100 plus streak and speed bonuses toward a leaderboard. Skip it if you want hints or an error counter: a wrong digit only turns red. An Easy grid is a few minutes, and the site's own speed cutoffs stretch to 23 minutes on Very Hard.

A given digit is printed plainly and cannot be changed, your own entries appear in the accent colour, and a digit that clashes with a matching one in its row, column or box turns red; reds must be cleared before the grid counts as solved. The LEFT counter above the grid counts down the empty cells, and Notes mode pencils small candidates that do not count as entries. There is no hint button and no mistake counter. The score arrives at the end: 100 points for the solve, one point per day of streak, and one speed bonus of 35, 20 or 10 points against the tier's cutoffs.

Easy

The most generous board: 62 given digits and 19 blanks, which the site says singles alone will usually solve, with speed bonuses at 1:30, 2:20 and 4:05.

Medium

The middle tier; the site does not publish its number of givens, and its speed cutoffs sit at 3:00, 5:00 and 9:00.

Hard

The tier where, in the site's words, box-line reduction quietly cracks most grids; speed cutoffs at 4:50, 8:40 and 13:40.

Very Hard

The most stripped-down board, 35 given digits, where singles are only the opening act; speed cutoffs at 8:00, 14:00 and 23:20.

Archive

The calendar button in the top bar opens past dates, with a Back to today link to return to the current puzzle.

• Scan by digit rather than by cell: take a number and sweep the board for boxes where its row and column neighbours leave exactly one legal cell. This cross-hatch is the method the site places at the top of its own tips.
• Learn to see naked singles as you go: when a cell's row, column and box rule out eight digits, the ninth writes itself. On Easy, with 62 givens, singles alone usually carry you to the end.
• When singles dry up, hunt pairs: two cells in one unit that share the same two candidates lock those digits between them and sweep them from every other cell in that unit.
• On Hard grids, box-line reduction does most of the work: if a digit's only homes inside a box lie along one row, it is spoken for in that row everywhere outside the box.
• Keep your notes honest. The site warns that a stale candidate is worse than none: note fully and prune after every placement on the two hardest levels, and skip notes entirely on Easy.
• Never guess. The site calls the red conflict flag a seatbelt rather than a strategy: every board has one solution reachable by reason, so back up and find the deduction you skipped.