A jazz band (jazz ensemble or jazz combo) is a musical ensemble that plays jazz music usually without a conductor. Jazz bands usually consist of a rhythm section and a horn section. During the jazz and swing eras in the mid-twentieth century, the most successful jazz orchestras also employed strings and harp in expanded arrangements, but their presence on the bandstand was more for visual impact and not as a key component of the ensemble, although the unique timbres of the strings and harp combined with the band as a whole provided an extra layer of ear-pleasing sonorities.