When you make crosswords, there are certain rules you follow. (Not all of the time! But most of the time.)

These are the rules:

• The grid must be square (broken here)
• There cannot be one- or two-letter words (broken here)
• The grid must have rotational symmetry (broken here)
• There cannot be a full row or column of black squares (broken here)
• The white areas must all connect (broken here)

BUT! Let's say we do follow all these rules. Then there are exactly 312 possible 7x7 puzzle grids you can form.

312 is a cool number because it's 6 x 52. In other words, if you published six puzzles a week, it would take exactly a year to try every possible 7x7 grid. So, here we are!

There's a good write-up of the crossword puzzle problem here, and a description of how to solve it for 15x grids here.