The underlying meter of “Do not go gentle into that good night” is iambic pentameter. Recall that an iamb is a metrical foot that consists of one unstressed and one stressed syllable, as in the words “com-plain” and “de-lay.” Iambic pentameter is traditional to the villanelle form, so Dylan’s use of this meter is not surprising. For an example of perfectly regular iambic meter, consider the second tercet (lines 4–6):

     Though wise | men at | their end | know dark | is right,
     Be-cause | their words | had forked | no light- | ning they
     Do not | go gen- | tle in- | to that | good night.

Thomas achieves this strictly measured iambic rhythm primarily with monosyllabic words. For instance, every word in the first line has just one syllable. Only four words in this tercet have multiple syllables: “because,” “lightning,” “gentle,” and “into.” It’s curious to note that, aside from “be-cause,” which is naturally iambic, all the other polysyllabic words are naturally trochaic. That is, these words each start with a stressed syllable and end with an unstressed syllable: “light-ning,” “gen-tle,” “in-to.” Even though Thomas has positioned these words in a way that maintains the lines’ iambic rhythm, the trochees resist the overall metrical flow. As such, the polysyllabic words resist the meter in a way that echoes the speaker’s call for their father to “rage against the dying of the light.”

Although iambic pentameter predominates throughout the poem, Thomas does introduce several irregularities to the underlying rhythm. Perhaps most significant is his frequent use of spondees, which are metrical feet that consist of two stressed syllables. Spondees appear consistently throughout the poem, and perhaps most obviously in the second of the villanelle’s two refrains (lines 3, 9, 15, and 19):

     Rage, rage | a-gainst | the dy- | ing of | the light.

Note that the opening foot has two stressed syllables. The use of a spondee at the beginning of the line creates a powerful suspension in the rhythm. The fact that Thomas constructs the spondee from the repetition of a single word causes the reader to slow down even more. In this case, Thomas effectively uses sound to echo sense, forcing a deferral of the rest of the line in a way that mimics the line’s call to resist the inevitable coming of death. Yet nowhere does Thomas use spondees more disruptively than in the third tercet (lines 7–9):

     Good men, | the last | wave by, | cry-ing | how bright
     Their frail | deeds might | have danced | in a | green bay,
     Rage, rage | a-gainst | the dy- | ing of | the light.

To partially balance the five spondees that appear in this tercet, Thomas includes two pyrrhics, which consist of two unstressed syllables. The metrical variation that results alternately speeds the poem up and slows it down in surprising ways.