Candidates and filters

Notes (pencil marks)

Switch the pad to Notes (or press N), select a cell and tap digits to write small pencil marks in it. Tap a digit again to remove it. Erase clears the marks of that cell. The marks are yours: type as many or as few as you like, for example only the digits of a Snyder notation. They stay visible without turning anything else on, and they are saved with the game and covered by undo.

Marks are drawn in a fixed 3 by 3 pattern, so a digit always sits in the same place within the cell. When you place a value, that digit is removed from the pencil marks of its row, column and box.

Settings → Clear pencil marks wipes all your marks, so you can start a fresh set.

Show candidates

Settings → Show candidates displays the candidates the solver keeps for every empty cell: all digits that no peer in the row, column or box already has. This replaces your pencil marks on screen while it is on, and Notes mode then edits those candidates instead, so you can remove the ones you have ruled out. Settings → Reset candidates restores them to everything the placed values allow. Hints work from these candidates, not from your pencil marks.

The digit filter

With the pad in Filter mode (F), tapping a digit highlights every cell that holds it or still has it as a candidate, and shows only that digit in those cells. Switching back to Value or Notes turns the filter off. It is the fastest way to see where a digit can go in each row, column and box. Tap the same digit again to clear the filter.

Shortcuts: Alt+1 to 9 toggles the filter from the keyboard; on a touch screen, press and hold a digit key for half a second.

Mark mistakes

With Settings → Mark mistakes on, a value that disagrees with the puzzle's solution is shown in red. It is off by default, because finding the error is often part of the puzzle.