A lyricist specialises in writing the textual component of songs — the words that are sung. In popular music, the roles of lyricist and composer (or songwriter) often overlap, with a single person writing both words and melody. In musical theatre and opera, the lyricist and composer are frequently different people: Oscar Hammerstein II wrote lyrics for Richard Rodgers' music, and W.S. Gilbert wrote words for Arthur Sullivan's melodies. A great lyricist balances multiple demands: the words must fit the melody rhythmically, they must be singable (certain vowel sounds work better on high notes), they must convey meaning and emotion, and they must be memorable. Lyricists work in many styles, from the narrative storytelling of country music to the wordplay of hip-hop and the poetic imagery of art song.