American-style crossword construction has a number of constraints, some bendable, some not.
- Every cell must be "keyed," i.e., part of a word Across and a word Down. Unkeyed cells are strictly forbidden.
- No word may be less than 3 letters. Two-letter words are strictly forbidden.
- The grid must be rotationally symmetric. (But this rule can be broken for fun. Bilaterally symmetric grids are relatively common these days. Totally asymmetric grids are very rare and always in service of some kind of fun — see https://www.xwordinfo.com/Thumbs?select=symmetry )
- No more than one-sixth of the squares can be black. (But this rule can be broken, usually either to make the puzzle less challenging by shortening the average word length, or to make the creator's life easier in order to achieve some other feat.)
- If a single black square is bordered on two adjoining sides by other black squares, then it could be turned white without destroying the other properties of the grid. Such black squares are called "cheaters" and are frowned upon. (Though they might serve a purpose, e.g. to fit a specific theme entry's length.)
