A villanelle is a highly structured poetic form consisting of nineteen lines, made up of five tercets followed by a quatrain. The first and third lines of the opening tercet are repeated alternately in the last lines of the succeeding stanzas, and then in the final stanza, the refrain serves as the poem’s two concluding lines. The rhyme scheme of a villanelle is ABA for the tercets and ABAA for the quatrain. The form has two repeating rhymes and two refrains. The villanelle originated as a simple ballad-like song with no fixed poetic form, often of a rustic or pastoral subject matter and contained refrains. The term "villanelle" was used to refer to a style of lyric verse that was similar to a ballad and did not have a fixed form prior to the 17th century. The defining feature of the villanelle is repetition, which makes it a challenging form to write. The villanelle has been noted as a form that frequently treats the subject of obsessions, and one which appeals to outsiders.