Shidoku is a variant of Sudoku, using the same rules, but played on a 4-by-4 grid. The game can be played with the Ace's through Four's of an ordinary deck of playing cards, with the same restrictions on numbers also applying to the suits. The App has three modes of play: A "Classic" Solitaire version where the player builds the grid from one dealt card, A Puzzle version where the player solves the grid with 5 cards given, and A Puzzle version where only the minimum clues needed to solve are given. The user may toggle to the Jack-Queen-King-Ace combination of cards if they wish. Points are awarded for each card played, based on speed and accuracy. There is an 'UNDO' button, but there is a penalty for using it. The puzzle versions allow the user to quit-and-solve, but doing so counts as a loss. A grid that was started but abandoned is scored as a loss. Switching to a different version or reloading before any card has been played has no effect on the statistics.