C Harmonica

• Even if it is not normally used, every player should have a harmonica in the key of C. The vast majority of instruction books and music are written for a C harmonica, making it a rather indispensable tool. Even if it is rarely used in actual performance, it is still an important practice instrument.
  
  

Straight and Cross Harp

• There are two common ways of playing the harmonica, each changing which key of harmonica is used. Straight harp has the root note as the blow note of the fourth hole. This root note is the same as the key. The harmonica notes form the major scale of the root note key.
  
  Cross harp is common in blues and rock harmonica. It uses the draw note of the second hole as the root note. This changes a harmonica to Mixolydian mode--the root note is the fifth of the key of the harmonica to be used. For example, a song in D played cross harp should be played with a G harmonica. Players need to adjust what key of harmonica they get accordingly.
  
  

Other Keys

• A solo harmonica player can play everything they want with a C harmonica, as long as they properly transcribe the songs. Players with a band need to get harmonicas to fit the key of the songs being played, which varies based on the band and style of music. Blues and rock players should definitely get harmonicas in A, D, and G. When playing in cross draw, the A harmonica is used in the key of E. The D harmonica is used for the key of A. The G harmonica forms D, which is less common in rock and blues.
  
  The third position (fourth hole draw note as the root) is useful for some songs with a more minor feel. The D harmonica is used over E minor songs. The G harmonica goes over A minor, which will be the most common use for it. The A harmonica forms B minor, which will not be as commonly needed.
  
  In other band situations, it's just a matter of finding which keys the band favors. It is much easier for a harmonica player to switch harmonicas than it is for a vocalist to change his range. There are rock and blues bands that have no songs in A or E, which means conventional wisdom will not apply.