Harmonica positions are basically how harmonica players describe the way in which they play different scales on a harmonica.
It's possible to play in different keys on a single harmonica. For example, you can take a C harmonica, and use it to play in the key of G. This would be called playing in 2nd position in the harmonica world. To get 2nd position- count up 5 from the key of the harmonica. Count up 5 from C, and you'll get G (counting C as 1).
Harmonicas come in different keys. However, the relative note layout on all of them is the same. So, if you learn to a tune on one harmonica, you can use the same blow/draw pattern on any other harmonica to play the same song/riff, just in a different key. This consistence of pattern makes the term "position" useful for communicating with other harp players to describe how you are playing a particular scale.
Playing in the key labeled on a harmonica is called playing in 1st position. So, playing in the key of C on a C harp is 1st position.
Playing in 2nd position generally involves using a lot of bent notes, and so it is often used for playing blues. 1st position is generally used for playing folk tunes.
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