I am having problem with the relation between melody and key. For example I don't understand when you play Simple Gifts melody you work with 2nd, 3rd and 4th string that has nothing to do with key of G! I am confused, what difference does it make, at the end you should play the 2nd, 3rd and 4th string, what is the connection to G key and how that can be helpful? Same for D key in Simple Gifts??
The Key of a song relates to the entire scale in which the song is being played. For instance, a song played in the Key of G is developed through the G major scale. A scale can be played using one string or multiple strings. That being said, chords are built from scales and consist of the I, iii, and V notes of the scale. Other scale notes can be found around the chord.
The melody line of Simple Gifts uses notes from the entire G scale or the Key of G. These notes consist of the I, iii, and V of the G chord and the notes (ii, IV and vi) surrounding the chord.