The following diagram shows how the three-line (A A B) and ...

[Mr.Thesaxman]

The following diagram shows how the three-line (A A B) and quatrain-refrain (a b c d A B) stanzas compare. Note the correspondence in the two forms between the first line A of the three-line stanza and the quatrain a b c d. Also note the correspondence between the last two lines of both forms A B.
  Three-line Stanza: : A A B :
  Quatrain-refrain Stanza: : a b c d A B :
 
  What will be an ideal response?

Question 2

Where does the two-line refrain occur?
 
  What will be an ideal response?

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Answer to Question 1

The chart below compares a three-line stanza from Poor Boy Blues (shown as P on the chart) and a quatrain-refrain stanza from She Got Me Walkin' (shown as S on the chart) during the twelve bars of a typical blues.

Answer to Question 2

 2-line refrain occurs within the last eight bars of the usual twelve-bar blues stanza, one text line during every four measures

[Mr.Thesaxman]

Excellent

[epscape]

Gracias!