Q: What chords are available on my diatonic?
A: In addition to playing some notes together that fit the background chords, there are three other major considerations -
Make sure your chording is in a range that doesn't clash with or upstage the soloist, singer, or other significant parts of the arrangement. This may mean choosing a lower-pitch harp or harps at times, and it means l listening closely to the entire band sound.
As already noted by some responders, there are certain rhythms that work well in certain contexts. What you're doing, in addition to plays chordal notes, is FINDING A RHYTHM PART -something that fits with, and adds something to, the groove already under way.
While keyboards tend to stick with rhythmic comping, both horns and rhythm guitar tend to work harmonized melodic lines into their rhythm parts. Again, you don't want to steal the spotlight, but some simple moving line, often in an abbreviated form, like punctuation, can be very effective. This was dealt within "Harmonica as Rhythm Guitar" in the Role Playing feature in HIP No. 1, with examples played on Cassette No. 1.
PARTIAL CHORDS -
George Miklas rightly notes the C, G7, B half-diminished (that's minor third, diminished fifth, and minor seventh) and D minor 6 available on the C diatonic. This gives us four of the seven triads (basic three-note chords) built on the notes of C major. If you narrow it down to two-note portions of chords, which can be used in moving lines, there are numerous other possibilities.
E minor 7th (E G B D) - E and G together in various blow holes, G and B in Draw 2 and 3, and B and D together in various draw holes for E minor 7 (or even E7, if you avoid playing the third, or bend to get it).
A minor (A C E G) - the entire blow chord is an A minor seventh chord, minus the root.
F major 6th (F A C D) - F and A draw notes are part of this chord, and D can be added as the sixth in many cases, giving you three notes of an F Major 6 chord.
Adding these three to the four that George named gives you chords built on all the notes of the C major scale. But there are more.
Bb Major seventh. (Bb D F A) - D and F are basic members of the Bb major triad, while A adds the major seventh. These are found together in Draw 4,5,and 6. Draw1 and 2 (D and G) are the third and sixth of Bb, and can be used as part of a Bb chord in some cases.
Eb Major 6th (Eb G Bb C) Eb Maj 9 (Eb G Bb D F) - there are several possibilities here. While neither Eb or Bb can be found in chord-able form on a C diatonic, several of its color notes can. C is the sixth, and can be played with G, the third, in some circumstances (which ones? Experiment and let your ear decide.) D and F are the major seventh and ninth respectively, extensions of the basic triad, and can be played together, perhaps even in alternation with C and G.
Taking this further means getting into some pretty strange chords, which I won;t get into here. But there are a couple of other fairly straightforward possibilities.
Symmetrical chords are chords built with only one kind of interval. The two most common are augmented chords and diminished seventh chords.
Augmented chords are built with major thirds - like C E G#. Now, which note is the root of that chord? It could be any one of the three - it has three potential identities. That means that any major third could be part of at least three augmented chords. C and E in Blow 1 and 2 (and elsewhere), G and B in Draw 2 and 3, and F and A in Draw 5 and 6 (and 9 and 10) all have this possibility.
The most common use for augmented chords is as a kind of fruity V chord, especially in gospel music and, with a more bumptious flavor, in swing music from the '30's.
Diminished seventh chords are made up of four notes, all a minor third apart. B D F Ab is one of them (The Ab, a diminished - smaller than minor - seventh is what distinguished this from the "half-diminished" B chord found on the draw notes, which sports A, a minor seventh in relation to B). Again, any note in the chord could be the root, so any portion of this chord - or even any minor third interval - could function as part any one of four diminished seventh chords. So that B diminished triad - B D F could also be part of D, F or Ab diminished.
Diminished chords are often used as a chord leading from IV back to I - as in C - C#dim7 - G
but can also be superimposed over regular seventh chords. We could take that B seventh chord and play it over a G7. B, D, and F would already be part of the G7, and Ab would add a minor ninth. Of course, this would also work over Bb7, Db7, and E7.
Did I say straightforward? Well, they are useful, and you will encounter them if you get even a little beyond basic I - IV - V blues. -- "Chords and Horns" 9 Dec 93 WY
(FMI: "SCALE BASICS" 20 Oct 93 WY) (FMI: "Re: Improvisation" 3 Feb 93 NL, GMa, several others) (FMI: "Re: Favorite harps" 1 Jul 94 CM) (FMI: "levin.txt" 21 Dec 93 GJ) (FMI: "11th position" 16 Mar 95 WY) (FMI: "Practice, Practice" 7 Dec 93 WY)