The screen angles used commonly in photographic color separation, the respective angle at which the halftone dots for each process color interfere with each other less and eliminate undesirable moiré patterns. These angles are: yellow at 0o from the horizontal (or, in other words, perfectly horizontal); magenta at 15o from the horizontal; black at 45o; and cyan at 75o. Digital halftone screening systems have trouble with the 15o and 75o angles, as their tangemts turn out to be irrational numbers. Consequently, a variety of approaches to the problem of these irrational screen angles have been devised. See Screen Angles.