In basic, bare-bones blues, the main musical theme is expressed in the first four-bar line, then repeated again in bars five through eight. The closing four bars usually cap the original theme line by putting a twist on it or summing up the original statement. And that adds up to 12 bars (or measures).

A musical theme is simply the melody that distinguishes one tune from another. The lyrical theme is the story line. And thyme is an herb.

To get an idea of how this works, take a look at the following example, a typical bar count for a standard 12-bar blues verse. To hear the rhythm, count each number out loud. Below each metered line are the lyrics, which state a basic theme that's repeated in the second line, and then wrapped up in the closer.

1-2-3-4, 2-2-3-4, 3-2-3-4, 4-2-3-4

I woke up this morning, feeling oh so sad . . .

5-2-3-4, 6-2-3-4, 7-2-3-4, 8-2-3-4

I woke up this morning, feeling oh so sad . . .

9-2-3-4, 10-2-3-4, 11-2-3-4, 12-2-3-4

Thinking about my baby and it makes me oh so sad.