Villanelle Form

A villanelle poem contains 19 lines:  Five tercets (3-line stanzas) followed by an ending quatrain (4-line stanza).

The first and third lines in the first stanza are repeated.

First, the rhyme scheme. There are only two rhymes in the Villanelle expressed by the letters 'A' and 'B'.

ABA   (Stanza 1)
ABA   (Stanza 2)
ABA   (Stanza 3)
ABA   (Stanza 4)
ABA   (Stanza 5)
ABAA (Stanza 6 Concluding Quatrain)

Second, the refrains.  The first and the third lines are refrains that are repeated four times each. Lower case ‘a’ and ‘b’ represent new lines. Referring to the first line as A and the third line as A3 for the refrains, the pattern would be:

A b A3    (Stanza 1)
a b A      (Stanza 2)
a b A3    (Stanza 3)
a b A      (Stanza 4)
a b A3    (Stanza 5)
a b A A3 (Stanza 6 and concluding quatrain)

Notice that the first and third lines alternate as refrains throughout the villanelle until the final quatrain in which both refrains appear. One of the best examples of the villanelle is Dylan Thomas’s  “Do Not Go Gentle Into That Good Night."

Source: Poets.org Villanelle form 

Unfortunately, they do not allow access from a blog link so I suggest doing a google search.

The Poetry Foundation