Diatonic harmonicas come in every key — C, D, E, F, G, A, and B, as well as in some of the half steps in between. The deepest-sounding normal-size diatonic harmonica is the G harp, and the highest sounding one is the F harp. Note that each of the keys will play slightly differently from the others. For example, the reeds will feel tighter on the higher-pitched harps and looser on the lower-pitched ones. This means that more air is required to play the lower-pitched harps. Also it will be harder to get certain bends to work on the higher-pitched harps.

The C harmonica is the recommended harp for beginners. Falling right in the middle of the pitch range of harmonicas, meaning that there are three harps with lower pitch (G, A, and B) and three with higher pitch (D, E, and F), the C harp does not present the challenges of using the lowest, and therefore loosest, or the highest and tightest reeds that exist. Rather it gives you a comfortable range of easy-to-use reeds. Another advantage is that the C harmonica plays in the keys of C and G, which are the keys of many commonly played folk songs. In addition, C harmonicas are the easiest to find in any store that sells musical instruments, whereas finding harps in the other keys can be tricky.