Richter tuning

Richter tuning is a layout plan for the notes of a diatonic instrument with multiple free reeds, such as a harmonica or accordion; the plan concerns which notes should be activated by each key. It is named after Joseph Richter, a Bohemian instrument maker who adopted the system for his harmonicas in the early 19th century, and who is also credited with inventing the mechanism that allows the harmonica to play different notes when the air is drawn instead of blown.
Richter tuning is designed as a compromise allowing players to produce both melodies and basic chords on diatonic instruments, despite their limited number of holes or keys. The lower portion of a Richter-tuned harmonica is designed to play the tonic and dominant chords on the blow and draw respectively. The remainder of the instrument is tuned to the notes of the diatonic scale, such that the blow notes are the members of the tonic chord, in sequence:
and the draw notes are those that are not members of the tonic chord, in sequence:
For example:
The above diagram shows that Richter tuning intentionally omits certain notes, particularly that the A and F are absent from the lowest octave. By comparison, solo tuning includes all the major scale notes for all octaves.

Variants

There have been many variants of Richter tuning.

Country tuning

Country tuning raises the initial F by a semitone to an F. This primarily aids in harmony in the key of G, facilitating easy play of the G-C-D chord progression, while maintaining a partial G7 higher on the harmonica. It can occasionally be helpful in some melodies, most notably "The Star-Spangled Banner," which has a low F# and a high F. For example:

Harmonic minor tuning

Harmonic minor tuning is a variation in which E is replaced by E and A is replaced by A. Thus the blow notes repeat a sequence of
and draw notes at some point begin to follow a repeating sequence of
though perhaps with a different initial sequence.
For example:

Major seventh tuning

Major seventh tuning raised each F by a semitone to F♯. For example

Melody Maker tuning

Melody Maker tuning raises the third Blow note by a full note, and raises the #5 Draw by a semitone to F. For example:
Note: the Melody Maker tuning is key labeled from the Draw Second Position.
The example above shows a Melody Maker in the Key of G.
The Melody Maker tuning was developed by Lee Oskar Harmonicas in the early 1980s.

Natural minor tuning

Natural minor tuning is a variation in which E is replaced by E and B is replaced by B. Thus the blow notes repeat a sequence of
and draw notes at some point begin to follow a repeating sequence of
though perhaps with a different initial sequence.
For example:

Paddy Richter tuning

Paddy Richter tuning. The tuning raises the 3 blow by a whole step e.g. from D to E on a G harp; this addition of the sixth also allows melodies on a harmonica's corresponding minor scale to be played an octave lower than would otherwise be possible without bending. The addition of this, however, adds an additional complication to playing harmonic music on the harmonica, requiring the 3 hole to be blocked when playing a tonic chord. The lower octave requires a bend on the 2 draw to achieve a missing note, which is also required on standard Richter-tuned harmonicas.
For example:

Richter Extended tuning

So-called Richter Extended tuning is in fact a significant departure from Richter tuning. In "Richter extended tuning," all Fs and As are removed from the instrument, and the dominant chord is, like C on the blow, extended through all of the holes on the draw, following the sequence
For example:

Powerbender tuning

Powerbender tuning was created by Brendan Power and utilises the standard Richter layout on the first four holes, but modifies holes 5 to 10 to enable more useful draw bends on all of the upper register notes.
This tuning also negates the need for overblows to achieve the blues scale throughout the whole two octaves of the harmonica, which would be required on a standard Richter tuned harp.
For example: