The 'Key' of G

A key is a combination of scale and chords which has a tonal center. This tonal center is the target note towards which other notes and chords will lead.

The key of G is derived from the notes of the G major scale:

The F♯ is found in the scale to ensure that all seven alphabet letters appear and that the major scale formula is preserved, namely the distance between one note to the next:

The half step of the major scale is always found between 3 and 4 and 7 up to the octave tonic, G.

These seven notes of the G major scale are the Root notes for the seven chords in the key of G. 

These are used to form chord progressions that will feel at rest upon the G chord. 

G, being the tonic is like the music’s ‘centre of gravity’.