Fifteen
Where must the empty square finish in Fifteen?
In the standard 4 × 4 puzzle, put 1–15 in reading order and leave the bottom-right square empty. A legal slide uses the empty space; tiles cannot jump over each other.
Worked example
The final row is 13, 14, 15, empty. A correctly ordered top row does not guarantee that the lower rows are solved.
What to check on your board
Apply the rule above to your current board, then verify the remaining puzzle constraints. The example explains a local deduction; it is not a solution to every randomly generated board.
Checking your next move
Identify the empty square before planning a sequence, then trace how each legal slide moves that empty space. Recheck the full reading order at the end, including the last row and the empty corner. A tile in the right place can still need to move while the remaining area is arranged.
Using this reference
For Fifteen, read the rule and the worked example together before opening the game. Separate what the rule explicitly requires from what only seems likely on the current board. A useful next move is one you can explain from the visible constraints, not merely a pattern that looks familiar.
If you get stuck, isolate a smaller part of the position and list what is still possible there. After a move, check the affected rows, regions or connections again. The reference explains the documented rules; progress, controls and save behavior belong to the linked game.
Read the developer’s manual ↗ · Open the developer’s browser version ↗
Manual inspected: · Simon Tatham & contributors · MIT