The lowest key harmonica you can get is in the key of G. The highest key is F (which is extremely shrill). A few manufactures have begun issuing the key of F in a lower octave which is a huge improvement. These are usually called Low F.
People email me all the time asking in which key a song should be played. It is not really that important. You can play any song in any key. I prefer to play in a lower key as it produces a much more mellow sound. If the tab numbers are up in the 7 - 10 holes, then I will use a G or A harmonica. If I use a C harmonica on these songs the sound is quite shrill and irritating. If the tab numbers are down around 2 - 5 then it will sound just fine on a C harmonica. When I throw a harmonica in my pocket, which I always do, it is usually the key of A. Almost any song will sound pleasant when played on an A harmonica.
The only time it is important that you have a particular key harmonica is when you are playing with other musicians. I played along with some guitar players who played everything in D. I guess that was the only key they knew. So I only needed to bring my D harmonica. Most good musicians can play songs in any key so you will need an assortment of harmonicas. You also need to know ahead of time in what key they are playing. If they change keys in the middle of a song you will have to have a second harmonica available in your pocket so you can make the change in the midst of the song.
It is important that you keep in mind that you can play any song, regardless of the key in which it was originally composed, on any diatonic scale harmonica. If the only harmonica you have is in the key of A, then every song you play will be in the key of A. And if you pick up a harmonica in the key of D and play the same numbering system, you are playing the song in the key of D.
Down In The Valley
Happy Birthday To You