**I am confused about the 12 bar blues. I know it is a chord progression. I know there are 12 bars and each bar has specific chords assigned to it. I have been told that if I don’t want to use the cords I can use arpeggios or the first note of the cord in the bars. However when I look at tabs from other people for the 12 bar blues it sometimes doesn’t appear that they are using format at all. When I inquire I get confusing answers including being told that what defines 12 bar blues is just the tracking music only! That must be an incomplete answer because people play the 12 bar blues for harmonica solos… When I play the 12 bar blues on my harmonica should I just be playing chords only? Should I just use the first note of each chord (I think this would be very limiting)? Can I put extra notes in that are not part of the cord (Doesn’t make sense to me but I’ve been told one can do this)? In other words can you please define for me the limits and boundaries of the 12 chord blues and specifically which rules do I need to obey to make good 12 bar blues music on my harmonica.
Thank you
Wayne. Email: wayne.trebbin@gmail.com
Hi
12 bar blues is to me like a format. There are heaps of great YouTube videos on it. @Hogie.Harmonica Hogie will I’m sure be able to answer your question however you will need to study the theory of chords a little more to fully grasp it. https://youtu.be/BbJCep7HZ_A?si=hh1Vcvem9QB8NOY0
I like this one as a starting point. Luke also has excellent videos
If you’re comping (accompanying others), then chords or single notes from the chord for that bar might be most appropriate. I wouldn’t say you can’t play other notes, but be considerate, especially if someone else is soloing. You can also compare to other instruments: just because a bass player is following the chord progression, that doesn’t mean they only play the root note (but that’s a totally valid bass line too).
If you’re the one soloing, then go wild. Chords, single notes, trills, whatever. If it doesn’t sound good, do something different next time. 
I guess this isn’t specific to playing 12-bar blues.
To contrast what I said before about what to do if something doesn’t sound good, sometimes it’s interesting to make intentionally discordant sounds. Or modify a scale to include a note that wouldn’t normally be in the key (see “blue notes”). In the right place, the wrong note can sound great.
@Wayne1 welcome to the forum! @john3 has given you a GREAT answer. The 12-bar blues is a chord progression. As harmonica players there are MANY different things we can do over it! Hope that helps. Rock on! 
I appreciate the responses to my query but I still remain confused on one point. I realize that 12 bar blues can be used for tracking music but when it’s being used for a solo and, as John 3 suggested, the soloist “goes crazy” playing all sorts of things what rules must be adhered to in that situation for the song to remain consdeted to be 12 bar blues. In other words what rules cannot be broken without the solo falling out of the class of 12 bar blues.
My opinion on the matter is to stay within the 12-bar format. In other words, don’t lose count of where you are in the overall phrase. Even that’s not hard and fast, since you might, for example, play a lead into the next 12-bar phrase starting on the last beat or so of the previous phrase.
As far as what notes to play, there’s a lot of room to experiment. A song that just came up while I was looking for examples is “Train, Train” by Blackfoot. Guitar solo at 1:39, then harmonica solo at 2:04, and then back to the verse. It sounds to me like the chord progression changes under the harmonica solo, but they stick to the phrasing (guitar for 12 bars, harmonica for 12 bars).
Generally,
The chords of a 12 bar blues are all part of a Key. A key is a collection of notes.
In a blues blues in a major key, you can rely on the major pentatonic scale of that key. But you also need to throw in some notes from the blues scale.
In a minor blues, just use notes from the blues scale or minor pentatonic.
More complex approaches are available, but not needed until you are farther down the line.
@Wayne1 of course as a soloist you have freedom to deviate, and many players may deviate by choice or by accident.
Whammer Jammer is a 12-bar blues format but Magic Dick fails to adhere to it strictly in his solo intro. He does 4 bars of the I chord and then only I bar of the IV (instead of 2) but eventually he obviously plays the V chord so the band can hear where he intends to be.
I recently saw an old video of BB King giving an interview where he talks about that he struggled keeping the form when a band wasn’t playing - he might add a bar here or there.
From what I’ve been told, the pre-war blues the form was a bit more free like that. It might be 13 bars instead of 12 or what have you. Slim Harpo’s “I’ve Got Love If You Want It” comes to mind here, which has 2 extra bars of I chord at the end, making it a 14-bar blues:
My understanding is that in the 40’s and 50’s the 12-bar form was gradually codified or adopted or whatever you wanna call it, so that all musicians could get together and play without even needing a rehearsal. Very handy indeed!
Hearing the 12-bar blues in your head while you’re playing can lead to more interesting playing perhaps than if you’re just doing the I-chord jam in the style of the Fox Chase or Lost John, although those styles I think can be pretty exciting as well, and hasn’t James Brown proven once and for all that you only need one chord? 
So I think strive for hearing the 12 bar blues if that’s what you’re going for when you’re playing solo, and don’t sweat it if you deviate a little bit. I’ve never any heard anybody say, “wow I really would love Whammer Jammer, except for the fact that he deviates from the 12 bar form in the intro!”
Remember in the final analysis there are no rules. This is art!
Yet, athe same time, constraints can actually breed increased creativity.
Hope that helps. Rock on!