Consonance occurs in poetry when the poet uses the same consonant sound repeatedly, often on stressed syllables and at word ends. In line one, Dylan Thomas uses a repeated "t" sound in not, gentle, into, that, and night. The effect is percussive and adds emphasis to the line's effect as an admonition. In line two, there is consonance present in the repeated sound of "d" in old, should, and day. It should be noted that the d's and t's are present in both of the first two lines, which intensifies the sound effect.

Assonance occurs in poetry when the poet uses the same vowel sound repeatedly, often on stressed syllables and at the end of words. There is less assonance employed in these two lines than consonance. The "o" sound in do, into, and good is the same. It should be noted that there are other uses of the letter "o" in the line, but they are sounded differently; for example, in "not" and "go." In the second line, Thomas uses a repeated long "a" sound in "age," rave" and "day."

The first two lines are

"Do not go gentle into that good night,
Old age should burn and rave at close of day;"

For analysis of poetic devices such as consonance and assonance, it often helps to read the poem out loud.  Consonance is represented by the repetition of the "t" sound in words such as "not," "gentle," "into," "that" and "night." Notice that for the most part the "t" sound is the end sound of the word or near the end.  You could also argue for "go," "gentle," and "good" in the first line because repeat a consonant sound, but those examples are more specifically alliteration (repetition of initial sounds in words close together).

For assonance, Thomas relies more heavily on that in Line 2. Notice the repetition of the long "A" sound in "age," "rave," and "day."

Keep in mind that when you identify patterns such as the use of consonance or assonance, you need to ask yourself why the author uses the pattern.  Is he simply trying to show off? In most cases, there is a better analysis.  In this poem by Dylan Thomas, the consonance in Line 1 not only provides emphasis to certain words, but it also gives a sense of finality because Thomas is discussing death.

In Line 2, the poet's choice of words with the long "a" sound causes the words to be drawn out longer--just as Thomas wants one to draw out his or her "age," "raving," or "day."