It's called the 12 bar blues because it's three statements that are four bars each. We have an initial statement, then there's a restatement, where the original melody gets stated in a slightly different way. Then our third statement of the 12 bar blues acts as an answer. When you put them together, you get the 12 bar blues. So our recipe is that, we're going to have 4 bars of A and we're going to have 2 bars of D which we call 4 because it's the fourth note of an A scale. Then it goes back to A for 2 bars and then we get our 5 chord E because it's the 5th note of the A scale and we get E, D and back to A to round it up. And that's a 12 bar blues form on a bass guitar.