here are multiple types of crosswords. The standard rules of crosswords for American style crosswords are: A square grid (popular sizes are 15x15 for weekdays and 21x21 for Sundays) An odd number of rows and columns, so that there can be a central row and column 180° rotation symmetry for the black/white squares: if you take the empty grid and rotate the page 180°, you should end up with the same pattern Words are three letters or longer Every letter must be checked, meaning it is part of both an Across and a Down entry Each answer is "common" (though there is such a thing as "crosswordese" which appears, but is frowned upon (see Crossword Puzzles: What are some examples of crosswordese? ) There can be a unifying theme for the longer answers, but this is not needed. Some more exotic crosswords which break the above conventions for a specific purpose (not just to get something to 'fit') sometimes appear. British style crosswords, sometimes cryptic crosswords, have a much lower level of interlocking than American style. They will often have an Across entry on alternating lines and a Down entry in alternating columns, so only every other letter is "checked". This adds an element of difficulty beyond that in the cluing. The numbering scheme is simply that every first letter of any answer needs to have a number assigned to it. The numbers start at top left and continue across the first row, then across the second row, working downward.