Details

Although this Demonstration allows a lot of freedom in designing crossword grids, traditional crosswords, such as those found in The New York Times, generally follow some basic guidelines:

1. They have rotational symmetry.

2. Fewer than one-sixth of the squares (38, in the case of a standard 15×15 grid) are black.

3. There are no unkeyed letters (each filled-in letter in the grid must be part of two words, one across and one down).

4. Words must be at least three letters in length.

5. The grid must be connected (the black squares cannot cut the grid into two or more separate pieces).

6. The number of words should be in the low 70s at a maximum.

Further details and guidelines for crossword construction can be found at the Cruciverb website [1].

Reference

[1] Cruciverb.com. (Dec 29, 2015) www.cruciverb.com/index.php?action=ezportal;sa=page;p=21.