A romance novel is a literary genre eveloped in Western culture, mainly in English-speaking countries. Novels in this genre place their primary focus on the relationship and romantic love between two people, and must have an "emotionally satisfying and optimistic ending." Through the late 20th and early 21st centuries, these novels are commercially in two main varieties: category romances, which are shorter books with a one-month shelf-life, and single-title romances, which are generally longer with a longer shelf-life. Separate from their type, a romance novel can exist within one of many subgenres, including contemporary, historical, and paranormal.