Sequencing licks to the IV7 Chord with a Bb (3') in them
A student just asked a very specific question about accompaniment playing. In Blues Harmonica Accompaniment Playing... the chapter on Hooks... he asked why I stated that a lick on the I7 Chord that contains a flat-3rd (B-flat, which is 3') does not sequence (transpose) to the IV7 well. He was also curious why we would want to use B-flat at all, since B is the 3rd of the chord, not Bb. Here's my answer...
There are three levels of dissonance, or how well notes we play match...
1) Chord Tones - Called non-active tones due to the fact that they match and don't need to be resolved = G B D (C Harmonica, 2nd Position) - Note the flat-7th is not included, it's dissonant (bluesy). These notes are used a lot for accompaniment playing due to the fast that they're very neutral... they don't grab attention... which is the job of the accompanist.
2) Non-Chord Tones - Called active tones... not of the chord, but of the scale of the chord = A C E F# - These offer slight dissonance when played and help to lead to chord tones. These can be used in accompaniment, but should not be held upon.
3) Outside Tones - These are not of the chord and not of the scale of the chord = Ab, Bb, Db, Eb and F and are very dissonant and aggressively want to resolve (to a scale tone, chord tone and ultimately the root note G). These are rare in accompaniment... used in your playing like a VERY spicy element... in moderation it can help it to sound exciting, but a tad to much will make you stand out too much and in some cases just sound plain bad.
Music is all about tension and release... how much tension is up to you. You should use item 1 most of the time, item 2 less and item 3 even lesser. Flipping this order will make your music very aggressive... the wrong approach for accompaniment playing. All of these notes are viable, it just gets down to the order in which you play them and how you organize them into a lick/phrase.
As for the Bb (B-flat) on the I7 Chord question. A hook is not there to just match, it's there to sound cool. With this in mind, a hook is a cool lick, not a simple chord structure... the player can use notes outside of the chord as well. For example... 2 2 2 3' 4+ hook played in "Help Me." 2 is the root, 3' is the flat-3rd (blues presentation of the 3rd) and 4+ is the 4th (normally an awkward note... it can be very dissonant... but in this lick it works).
To sequence (transpose) this lick to the IV7 Chord it would be... 1+ 1+ 1+ N/A 2" or 4+ 4+ 4+ N/A 5 or 7+ 7+ 7+ 8'+ 9... N/A being Not Available (unless you use overbends). You can see how the Bb (3') on the I7 Chord does not sequence well to the IV7 Chord. So, as a rule of thumb, we don't sequence licks with 3' in them.
For further information on this subject visit Improvising Study #8 and Improvising Study #9 at BluesHarmonica.com.