Sudoku Corner, Centre and Colour Marks Explained
Every sudoku player eventually needs somewhere to write a number down without committing to it, and that is where sudoku corner marks, centre marks and colour come in. Our sudoku controls give you four separate ways to mark a cell instead of one generic pencil mark, because a small digit in the corner of a cell means something different from the same digit in the centre. Corner marks place small candidate digits into the corners of the cell instead of scattering them across it, centre marks keep candidates together in the middle of the cell, and colour lets you shade a cell instead of writing anything into it at all. All four modes read the same keyboard, mouse and touch input, so once you know how selection works, switching between Normal, Corner, Centre and Colour is a single key press.
Selecting one cell or several at once
Before any mark goes into a cell, you have to select it, and our sudoku controls treat selection the same way whether you end up in Normal, Corner, Centre or Colour mode. A click or a tap picks one cell, dragging with the mouse or a finger across several cells picks all of them, and holding Ctrl while clicking adds a single cell to that selection or takes it back out.
Once cells are selected you can move the selection with the arrow keys, which wrap around the edge of the grid instead of stopping there, and WASD works as an alternative to the arrows. Escape clears the selection without touching anything you had already written. The given digits printed into the puzzle at the start can still be selected, for example to colour them or to place marks in the cells around them, but their own value cannot be overwritten.
The four entry modes: Normal, Corner, Centre and Colour
Normal mode writes the large digit that sits in the middle of an empty cell, the one you use when you are sure of the answer rather than noting a candidate. Corner mode writes small digits into the corners of the cell, Centre mode writes small digits into the middle of the cell, and Colour mode fills the whole cell with one of nine colours instead of a digit. You switch between the four with the keys Z, X, C and V, and the space bar cycles through all four in turn.
You do not always have to switch modes by hand. Holding Shift turns on Corner mode for as long as you keep it held, and holding Ctrl does the same for Centre mode, so you can drop into corner marks for one digit and return to whichever mode you were using the moment you let go.
Typing digits, clearing marks and undoing mistakes
Digits themselves go in with the number keys one to nine or with the numeric keypad; on a smaller grid you can only type up to the size of that grid. Backspace and Delete follow a fixed order in the selected cells: they clear a large digit first if one is there, then any corner or centre marks if there is no digit left, and only then the colour if nothing else remains.
Undo and Redo have no limit, and you can reach them with U, R, Ctrl+Z, Ctrl+Y or Ctrl+Shift+Z. Restart puts the given digits back the way they started and clears everything you added on top, but it is not a dead end: one Undo brings the whole grid back exactly as it was, and the clock keeps counting the whole time.
Default settings you can change before you start
Every sudoku on the site opens with the same starting settings, and all of them live in a menu you can reach before or during a game. Control method starts on Selection, meaning you pick the cell first and the digit second, Display timer starts switched on, and Highlight matching numbers starts switched off.
Default note style opens on Corner, so a fresh note lands as a corner mark until you choose otherwise, Auto remove restricted notes starts off, Auto pause is on so the clock stops itself while you are away, and Confirm before Restart is on so a stray key press cannot wipe a grid you were still solving.
Why Check never shows a red cell and you never truly have to guess
Nothing on the grid turns red while you are solving. The only way to learn about an error is to press Check, and it works in two steps: first a count of how many mistakes exist and roughly where they are, then, only if you ask again, the exact cells.
Hint works the same way in two steps of its own: first a sentence naming the technique to use and the general area, without giving away the value, then the highlight and the single move it unlocks. Both Check and Hint are free and unlimited while you play, they are only counted and shown together once you finish, and the timer on screen is a score you are trying to beat, never a limit that cuts your game short. That is possible because every puzzle you are given has exactly one solution, confirmed by the generator itself rather than by a person, and thrown away if a human solver could not finish it without a guess.
Frequently asked questions
What is the difference between corner marks and centre marks?
Corner marks sit in the corners of a cell and centre marks sit in the middle of it; they are two separate sets of small candidate digits, and Corner or Centre mode decides which set your next keystroke goes into.
Do the held Shift and Ctrl shortcuts replace the Z, X, C and V keys?
No, they add a shortcut on top of them. Z, X, C and V still switch you into Normal, Corner, Centre or Colour mode until you switch again by hand, while holding Shift or Ctrl only borrows Corner or Centre mode for as long as that key stays down.
Does pressing Escape erase my marks or my selected colour?
No. Escape only clears which cells are selected; it does not touch the digits, corner or centre marks, or the colour already sitting in those cells.
Try it today
Open today's puzzle and switch between Normal, Corner, Centre and Colour with a single key to see the difference on your own screen. Every puzzle behind that link still carries exactly one checked solution, and the same free, two step Check and Hint described above.
Sources. The facts in this note are numbered after our internal fact list and were checked on the publication date. ARLing is not a bank; a clean check result is no guarantee that the bank accepts the payment. If a source has changed since, write to andrej@arling.sk and we will correct this page.
- F110, F111, F112, F113: ARLing, štandard ovládania hier, časť Vzor A (overené 11. 9. 2026)
- F114: ARLing, spoločné pravidlá pre hry, časť Generátor a Súbory (overené 11. 9. 2026)
- F115: ARLing, štandard ovládania hier (overené 11. 9. 2026)