Genre classification is more complex than simply comparing a film to a checklist of features. Genres evolve over time as new films are produced and societal expectations change. For example, the film noir genre emerged in the 1950s without an existing checklist, and was later adapted into the neo-noir genre with a modern setting. Some films also blend elements of multiple genres, making classification difficult. As each new film aims to be original, genres have become more of a spectrum than rigid categories, making it inadequate to only use predefined checklists to determine a film's genre.