Hypothetically--very hypothetically:

You find some short (8-bar) patterns in early "blues" from around the [Mississippi] Delta and New Orleans.

"Baby, tell me where'd you sleep last night?
You wasn't here an' I know it ain't right."

Then it went up the Mississippi River and somewhere about half-way to Canada sprouted a third line, called the 'sting in the tail.' This is either "Kansas City Blues" [e.g., "Kansas City"] or "Chicago Blues." Many times the first two lines had the same words. Four counts each, here it is in A:

A D7 A A7
D7 D7 A A
E7 D7 A E7

Then later [?] that got squared off again and you got a 4-line "16-bar" pattern.

[Lonesome's got another 12-bar up there.]

I am now prepared to be enlightened. :)

CC

PS. Hope the line breaks are line breaks