SPACINGPart of the ease of reading and rhythm of the screenplay is the spacing. The scene description is single spaced as are the dialogue blocks. Everything else is separated by double spacing. The only exception is when three blank lines are used before a Scene Slug after a scene which had no Scene Break (i.e.: no "CUT TO:" or "DISSOLVE TO:").
This is not a hard and fast rule and many writers will put in extra blank lines or, rarely, squeeze a blank line out to keep a page as a unit. More will be said on this below under page cutting.