Good Old Sudoku

The sudoku you already know. Nothing more, and nothing missing.

Four difficulties — Easy, Medium, Hard, Expert — and you pick. Difficulty is graded by which technique a board demands, not by how many clues it hands you, so choosing Hard gets you a board that is genuinely hard rather than an easy one with more blanks.

THE BOARD

Every puzzle has exactly one solution, and that is verified while the board is being made. There is nothing to guess at; every step can be reasoned out.

• Three mistakes — a wrong number is named wrong the moment you write it
• Three hints, and they open the most constrained cell rather than a random one, so a hint lands where you were already looking
• Unlimited undo and erase
• Pencil marks, and placing a value clears it from its neighbours' marks for you
• Tap a number and every twin on the board lights up
• A finished row, column or box is both seen and heard

THE CLOCK

Each difficulty keeps its own best time. Somebody who finishes Easy in two minutes takes twenty over Expert; one shared record would be set on the first day and never approached again. When you beat a record, it tells you what it beat.

The clock stops behind a panel and when you put the phone down. No board is lost to a phone call.

WHERE YOU LEFT OFF

Close the app, or let the phone drop it out of memory, and your board is still there: the numbers you wrote, your pencil marks, what you have left and the clock. It comes back put down — the clock waits until you tap.

LOOK

Light and dark, or whatever the phone is set to. The colours were measured rather than eyeballed: the given clues against your own answers, a wrong cell against the ground, a lit row against the cell rules — the distance between each pair is held by a test.

ADVERTS, PLAINLY

There is no banner over the board; the playing area is never covered.

A rewarded video only appears if you ask for one: to get another hint when yours run out, or to save a board when the third mistake goes in. Decline and you never see one.

An interstitial can only appear when you finish a board: never during the first five, and after that at most one board in three and no closer together than three minutes. Never when you lose.

A one-time purchase removes interstitials entirely. Rewarded videos keep working — what you bought was the interruption, not the help you asked for.

No account, no sign-in, no analytics. Your progress lives on your phone and one tap in Settings erases it. Adverts are served by Google AdMob.

NINETEEN LANGUAGES

Turkish, English, Russian, German, French, Spanish, Portuguese, Italian, Dutch, Polish, Ukrainian, Azerbaijani, Arabic, Persian, Indonesian, Hindi, Chinese, Japanese, Korean. In Arabic and Persian the whole interface turns right to left.