I knew I had seen this somewhere and found it in a book written by Steve Baker “The Harp Handbook” p 21
Although the Richter harmonica was originally designed to be played in the major key of the blow chord (ie in Cmajor on a C harp aka 1st position or “straight”) It is nevertheless possible (and usual) to play in several different keys on one harp. Depending on which of the available notes you take as the starting point, or root note, of your scale, a number of different modes are at your disposal. Harp players usually refer to these as “positions”; theoretically there are 12 of them, one for each note of the 12 tone scale.
First position or straight harp
C D E F G A B C
2nd position or Cross harp
Using a base C harp, the 2nd position or “cross” position will be in the key of G aka Mixolydian scale
This uses the C major scale starting and finishing on G. G is the 5th of the C scale. It is major apart from the minor 7th
G A B C D E F G
However as harp players we tend to bend some (two) of these and it becomes the classic blues scale
G Bb C C# D F G
Bb is a bent draw 3 note, C# is a bent draw 4
The 3rd or “double crossed” position
If you take the D (draw 4) as your root note you get the following scale
D E F G A B C D
This will be the D minor or Dorian scale, although the 3rd (F) and 7th © are minor the 6th (B) is major. Therefore this scale has both major and minor characteristics.
To make this more “bluesy” we bend the 6th (6Draw)
D F G Ab A C D
The minor (natural) scale or aeolian mode is the 4th position
In the fourth position A is the root note (3 draw) and bent
A B C D E F G A
Because the first or root note is bent, it is easier to get a minor scale harp which is ready to go as is.
Hope this helps but probably easiest is to think of it as:
All in the key of C
First position - straight = C
Second position - cross harp is the fifth of C = G
Third position - doublecrossed harp is the second of C = D
Fourth position - natural minor is the minor third = Am or A harp with the root note flatted
Personally if just use play louder and hit the root note at least once every 12 notes just to make sure I know where the heck I am. Any note that sounds wrong I play it another three times just so the audience think I did it on purpose.