A villanelle is a structured form of poetry consisting of 19 lines, divided into five tercets (three-line stanzas) followed by a quatrain (four-line stanza). The rhyme scheme is ABA for the tercets and ABAA for the quatrain. The first and third lines of the opening tercet are repeated alternately at the end of the subsequent stanzas and both appear in the final quatrain. This poetic form is known for its musical quality and emotional depth. One of the most famous examples of a villanelle is Dylan Thomas's "Do Not Go Gentle into That Good Night," which explores themes of mortality and resistance. Villanelles often convey complex feelings through their repetitive structure.