The poem is a villanelle, which has 19 lines; 5 tercets (3-line stanzas) and a concluding quatrain (4-line stanza). The tercets follow an ABA rhyme pattern: "Do not go gentle into that good night,/Old age should burn and rave at close of day;/Rage, rage against the dying of the light." The quatrain follows an ABAA rhyme pattern: "And you, my father, there on the sad height,/Curse, bless, me now with your fierce tears, I pray./Do not go gentle into that good night./Rage, rage against the dying of the light."
The interesting thing is that the A and the B rhymes stay the same throughout the entire thing; the poem is based solely on rhyming the words "night" and "day". And yet, Thomas manages to elicit such depth under the very specific and potentially limiting structures of the villanelle poem style.