Cosplay may be presented in a number of ways and places. A subset of cosplay culture is centered on sex appeal, with cosplayers specifically choosing characters known for their attractiveness or revealing costumes. However, wearing a revealing costume can be a sensitive issue while appearing in public. People appearing naked at American science fiction fandom conventions during the 1970s were so common, a "no costume is no costume" rule was introduced. Some conventions throughout the United States, such as Phoenix Comicon and Penny Arcade Expo, have also issued rules upon which they reserve the right to ask attendees to leave or change their costumes if deemed to be inappropriate to a family-friendly environment or something of a similar nature.