Avoidable Rectangle
Short answer: a Unique Rectangle where the pair has already resolved. Three corners of a two-box rectangle are filled in — and none of them was a clue, because you placed them yourself. If the fourth corner took the shape that completes the pattern, all four could swap and the puzzle would have had two answers. So it cannot.
What an avoidable rectangle looks like
This is the one technique on the site that depends on something outside the grid: which cells were printed and which you wrote in. Two identical-looking boards can have different avoidable rectangles, or none at all, depending purely on where the setter's clues were. That is why it is so easy to walk past — the pattern is made of solved cells, and solved cells are exactly what you have stopped looking at.
The word "avoidable" is about the setter, not you. A rectangle of four non-clue cells with only two shapes between them would have made the puzzle ambiguous, so a competent setter had to avoid leaving one — which means it is not there, which is what you get to use.
How to use an avoidable rectangle
- Look at solved cells, not candidates. This is the habit to build. Once you are deep into a solve, scan the cells you have filled in for rectangles — two rows, two columns, exactly two boxes.
- Confirm none of the three was a clue. Every corner you are relying on must be one you placed. A single given among them and the argument disappears, because the setter was free to leave that shape there.
- Check the shapes form the pattern. The corner diagonally opposite the empty one holds a shape; the other two both hold a second, different shape. That is the arrangement one swap away from a deadly rectangle.
- Strike the diagonal shape. Remove the diagonal corner’s shape from the empty cell. If that leaves one candidate, place it.
The same reasoning before the cells are filled is a Unique Rectangle, and it is much easier to spot because the pencil marks give it away. Both rest on the puzzle having exactly one solution — if you do not trust that, use neither.
Avoidable vs Unique Rectangle
Same theorem, opposite moment. A Unique Rectangle catches the danger while three corners are still pencil marks; an avoidable rectangle catches it after those three have been placed. That is also why this page has no "find it in your grid" tool, unlike every other technique here: a grid pasted as 81 characters cannot say which cells were clues and which you worked out, and without that the pattern is invisible. It is the one technique you have to spot with your own eyes.
Frequently asked questions
Why does it matter whether a cell was a clue?
Because the argument is about what the setter could have published. A given is fixed by the puzzle and can be anything; a cell you derived was forced by the clues. Only when all four corners are derived does swapping the pair produce a genuine second solution — which is the thing a proper puzzle cannot have.
How do I keep track of which cells were clues?
On paper, the clues are printed and your entries are handwritten. In form9 and most apps, entered shapes are shown in a different colour from the givens, so the distinction is already on screen — you only have to start noticing it.
Why is there no finder for this page?
Every other technique here can be found from the grid alone, so this site can look for it in a board you paste in. This one cannot: the 81 characters record what is in each cell, not where it came from. A finder would answer "not found" every time, which would be worse than not offering one.
Is it the same as a deadly pattern?
A deadly pattern is the thing being avoided — four non-clue cells holding only two shapes in a rectangle across two boxes, which makes a puzzle ambiguous. An avoidable rectangle is the deduction that comes from knowing the setter cannot have left one.