Monotype (name of an American typesetting machine) is a synonym for non-proportional fonts (Monotype fonts). In contrast to proportional fonts, monotype fonts have a fixed character width, which leads to different optical distances between the individual characters. Therefore, in monotype fonts, the distance between i and o (io) is larger than the distance between a and o (ao). Since, in appearance, monotype fonts differ significantly from proportional fonts, they are frequently used to mark e.g. source code examples in text documents.