How to Tell if a Rectangle Is Valid

That gets you to local validity. Then comes the stronger question: does the rectangle still leave the surrounding board in a workable state? This is where experienced solvers pull ahead. A move can pass the basic tests and still create impossible leftovers for neighboring clues.

In practice, you can think of a valid rectangle as one that survives both filters. It must satisfy the direct rules, and it must fit the structure of the rest of the puzzle. If a move feels legal but causes chaos everywhere else, it is often only temporarily plausible.

The Solver page is useful here because it makes candidate pressure visible. When you see how many legal rectangles each clue has, the difference between a clean move and a dangerous move becomes much easier to explain.