- Clue (number)
- A given number in the grid. It states the area of the rectangle that contains it.
- Rectangle (region, block)
- A solid rectangular group of cells that contains exactly one clue. Squares are rectangles.
- Area
- The number of cells in a rectangle: width × height.
- Option (candidate)
- One possible placement of a rectangle for a clue, consistent with the rules so far.
- Forced rectangle
- A clue with only one remaining option. It can be drawn immediately.
- Reachability
- Whether any option of a clue covers a given cell. A cell reachable by only one clue belongs to it.
- Overlap
- Cells shared by every remaining option of a clue. They belong to that clue even before its shape is known.
- Region counting
- Using the fact that a sealed-off area must contain clues that sum to its number of cells.
- Contradiction
- Assuming an option and showing it leads to an impossible position, so the other option must be true.
- Unique solution
- A puzzle with exactly one valid answer. Every PuzzleShikaku puzzle is checked for this.
- Nikoli
- The Japanese puzzle publisher that first published Shikaku in 1989 and popularised Sudoku.
- Shikaku ni Kire
- The original Japanese title (四角に切れ), roughly “divide by squares”.
- Prime clue
- A clue that is a prime number (2, 3, 5, 7…). Its rectangle is always a 1-wide strip.
See these ideas in action in the strategy guide.