Fifteen

Arrange the numbers from left to right, top to bottom, with the empty space in the bottom-right corner. A tile next to the gap slides into it. You can also tap farther along its row or column to slide several tiles at once; each tile shifted counts as a move.

By default, arrows move a tile in the direction pressed, so the gap moves the opposite way. Turn on “Arrow keys move the gap” to reverse this.

Suggested move (H) plays the upstream hint. Repeating it solves the puzzle, though it may scramble your progress along the way. Hints mark the game as assisted. Show solution immediately reveals an ordered board for practice. Assisted completion is labelled separately from an unassisted finish.

Undo and Redo preserve your history; Restart returns to the same initial board and clears assistance. Restart can be undone except at the history limit, where it begins a fresh history so you can keep playing the same puzzle. Completion stays recorded while you practise further slides; upstream hints become available again after undoing the finish or restarting. There are no pencil marks or erase actions in this sliding puzzle.