The villanelle is a 19-line poem with what is sometimes termed an "obsessive" structure, in which entire lines are repeated as refrains. The form itself is French and dates back to the middle ages, but it only became popular in English in the 19th century. Its rhyme scheme is essentially similar to terza rima, with the pattern somewhat complicated by the repeated lines. It may be written using any length of poetic line (there are free verse villanelles), but the most common is iambic pentameter, which is the meter used by Thomas.

The rhyme scheme is ABA ABA ABA ABA ABA ABAA.  The more complicated patterning is that of the refrains.

Let R1 be the first refrain, R2 the second refrain, and A and B the rhymes. the schematic for the poem would be as follows:

Do not go gentle into that good night,                    R1/A
Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx      B
Rage, rage against the dying of the light.               R2/A

Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx      A
Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx      B
Do not go gentle into that good night.                    R1/A

Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx      A
Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx      B
Rage, rage against the dying of the light.               R2/A

Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx      A
Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx      B
Do not go gentle into that good night.    R1/A

Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx      A
Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx      B
Rage, rage against the dying of the light.               R2/A

Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx  A
Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx  B
Do not go gentle into that good night.                R1/A
Rage, rage against the dying of the light.              R2/A