The 12 bar blues is one of the most common and well loved chord progressions. Knowing how it works lets you really work on playing the blues and making up your own songs very quickley.
The phrase 12 bar refers to the number of bars, or measures, that the repeatable pattern takes up. The twelve measures can be broken down into three four-bar segments.
The lyrics typically follow an AAB pattern, where the first two 4 bar stanza's are "A" and the third is "B". The first and second lines are usually repeated, and the third is a response to them, often with a twist.
Not all blues songs follow this pattern, but it is the most well known and easy to understand format, which will help the listener to understand the musical framework used in the blues better.
Before we get into more about how to sit down with your piano or guitar and do it, listen to an example of a 12-bar blues song:
( And would also be a fabulous example of how you're allowed to go completely insane while playing the 12 bar blues. Sometimes insanity is fun ;-)~)