In western music there are 12 keys to choose from (C, Db, D, Eb, E, F, Gb, G, Ab, A, Bb, B) and harps come in all these keys!
The blues harp is a diatonic harmonica which means that it’s fixed in a particular key (this is not entirely true, as we’ll see later!) so if your playing along with a song you’ll need to know what key harp to choose. Also different keys of harp will sound either higher or lower in pitch depending on which one you choose, the lowest harp is the G harp and the highest the F harp (although I have seen a Low F harp).
A Good Key to Start
A good key to start with would be a C harp, this is somewhere in the middle, not too high or low. In this case you’d be playing in the key of C in first position and G in second position (also known as cross-harp).
Other Common Keys
Other common harp keys include the A harp and the D harp, an A harp sounds lower in pitch than a C harp and a D harp higher in pitch than a C harp. All harps A, C and D are great choices if you want to play along with someone on guitar because in 2nd position they match the common guitar keys of E, G and A respectively.