When I became interested in playing bluegrass with my harp I ran into the accompaniment dilemma. Playing melodies is one thing, accompanying is another. At the suggestion of the fine folks on the Bluegrass Harp list I investigated what the other instruments do. One of them is to chop chords on the 2/4 back beats. That presents a few difficulties for the harmonica. The most common chords are the I, IV, and V chords. The ones that you want may not be there. The sections that follow cover the two most popular ways of playing Major music; 1st and 2nd positions, and ways of dealing with the dilemma.
The chart above shows the four (3 or more notes) chords that are available on the standard diatonic blues harp. It is commonly called a Richter tuning, but it was recently pointed out on Harp-L that Richter actually refers to the construction where each hole has one blow and one draw reed. Since the term Richter tuning is so commonly used for the standard Marine Band/Blues Harp tuning, I will go ahead and use that reference here.
To use the chart, choose the harp(s) on the left column that have the required chords listed in the table. For example; if you want to play the major I-IV-V chords while playing in the key of G, the little circle of 5ths shows I=G, IV=C, and V=D. The G and D are available on a G harp played 1st position, and the G and C are available on a C harp played in 2nd position.
Ist position seems to be a natural for playing Major music since the correct notes are there. Many bluegrass tunes are two chord songs with the I-V. 1st position gives you I chords all across the harp with blow chords, and it gives you one V chord at the 234 draw. There are two issues to be dealt with.
One, it is often desirable to play a V chord sometimes above, and sometimes below the I chord. To accomplish this you'd need a second harp with a key a 5th above the key of the harp that you are playing on. Harp switching for this might not be worth the effort since the second harp still would not allow for a IV chord and many tunes use three chords, the I-IV-V.
Two, to get a IV chord you need a 2nd harp with a key a 4th below the key of the harp that you are playing. Harp switching can work and is worth trying.
Three, you can play the 56 draw (1-3 interval) to imply a IV chord (easy to do), or use a big tongue split (5,9 draw) to play an octave substitute for the chord (slightly more difficult).
2nd position gives you one I chord (234 draw) and IV chords all across the harp (blow chords). 2nd position is quite popular for blues and rock. It gives you a Mixolydian scale (flat 7) so it's V chord is minor. You must avoid the 7 note, or be able to overblow (not a bend) to get the proper Major scale. If vamping major V chord triads is what you are after you have a some choices for that.
One, tongue splitting to remove the minor 3 from the chord (1-5 interval with 4,6 split) or tongue spit 4,8 for octaves.
Two, harp switching to a harp that is one 5th above the tonic (two 5ths above the one you are playing).
There's not a lot of information that is easy to find on playing with multiple harps, but there is an article on Pat Missin's very informative web page.
For people accustomed to playing 2nd position, the country tuning is a natural choice. You can make one by tuning the 5 draw note up a semitone. This gives you a Major scale in 2nd position, and a V chord at the 456 draw. You do need to bend notes at the low end to pick up two missing notes of the scale, but 2nd position players routinely do this. If you are not comfortable with the idea of retuning your own harp, Hohner sells a country tuned harp.
The V chord that you get with a country tuned harp is higher than the IV, if you want one that is below the I chord you need to make the same swap that you make with a Richter tuned harp.
The Melody Maker™ tuning by Lee Oskar goes a step beyond the country tuning with the "Paddy Richter" change to the 3 blow in addition to the country tuning raised 5 draw. This lets you play at the low end without needing to bend the 2 and 3 draw notes for a complete scale. The price is small, loosing the low blow IV chord. For players who normally avoid bending notes this is a nice set-up. To get a low V chord you still have to harp swap like with the country tuning.
Note: While Melody Maker harps are played in 2nd position, they are labeled that way. In other words. You don't get a C Melody Maker to play in G, you get one labeled G. More information about the Lee Oskar Melody Maker.
For the example shown above, we are playing in the key of C with a Melody Maker, or Country tuned harp so we have the I-IV-V on one harp. To get the ii-iii-vi chords we use a Natural Minor tuned harp in the relative minor key. For the key of C, that's A minor. Where the 2nd position i-iv-v for the key of A are located at, we have vi (Am7), ii (Dm), and iii (Em) for the key of C. Cool!