"Do Not Go Gentle into That Good Night" is written in a very specific form, the villanelle. Now follow us closely here, because we're going to hit you with more numbers than a baseball stats chart at the end of a great season.
Villanelles have nineteen lines divided into five three-line stanzas and a sixth stanza with four lines. In English, villanelles tend to be written in the common metrical pattern called iambic pentameter, which means ten syllables per line, with every other syllable stressed, starting with the second syllable. So the lines will sound like this: da-DUM da-DUM da-DUM da-DUM da-DUM. For example, the second line of this poem is, more or less:
Old age should burn and rave at close of day
The meter, however, isn't required in order to make it a villanelle – we just thought we'd mention it.
What villanelles are required to have is an intricate rhyme scheme and two lines that are refrains – like refrains in songs, they get repeated over and over (Beyonce anyone? "To the left, to the left, everything you own in a box to the left"). The rhyme scheme is ABA ABA ABA ABA ABA ABAA, so there are only two rhymes that end all the lines. In addition, the first line and third line, the refrains, are repeated four times each – the first line appears at the end of stanzas 2 and 4 and as the second-to-last line in stanza 6. The poem's third line appears again at the end of stanzas 3, 5, and 6. So if we call the first line A and the third line A', and any line that rhymes with them a, then the rhyme scheme is: AbA' abA abA' abA abA' abAA'.
Looks almost like math, doesn't it? That's because villanelles have to have a mathematical precision when they're written in English. The villanelle form wasn't designed for the English language, which has fewer rhyming words than many other European languages. Villanelles were originally a French type of poetry, and they only became popular in English as a late-19th-century and early-20th-century import. Dylan Thomas's ability to follow this strict and complicated form, which actually works against the language he's using, and still create such an emotional poem with an urgent feel, is truly impressive.