European based music is organized in keys, sequences of 8 principal notes selected from the 12 tones available in the chromatic scale.
http://en.wikipedia.org/wiki/Key_%28music%29
The key of G (major assumed) uses the notes:
G A B C D E F# (F sharp)
The choice of key then implies a set of chords, with the three main chords being G, C, and D.
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