Old school blues lyrics use a “call and response” approach to delivering the lyrics. You generally break a verse into three lines, each of which is a two-part phrase. The first two lines are either identical or very similar sounding (as far as the words themselves as well as the meaning). These are the “call” lines. You then “respond” with a single line that answers the call. Often, the last sound of the response rhymes with the last sound of the call, though often the rhyme is considered a “sloppy rhyme” with only partial sound similarity. Here is an example:

I’ve been downhearted / ever since I was born
(I said) I’ve been downhearted / ever since I was born
My clothes were always ragged /shoes were always worn

So where do these fit into the structure of the 12 bar blues song? The first line fits somewhere into the first four bars of the I chord (usually, you do a little jamming on the other bars), then the second line appears during the IV chord, and the response occurs during the V-IV walkdown…something like this:

Bar	Chord	Lyric
1	I
2	I (IV)	I’ve been downhearted
3	I	ever since I was born
4	I	(small lead)(I said)
5	IV	I’ve been downhearted
6	IV	ever since I was born
7	I
8	I
9	V	My clothes were always ragged
10	IV	shoes were always worn
11	I
12	V