Here we show the first measures of Mozart's Sonata K. 283 in G Major. This is a good example of how the I (tonic) , IV (subdominant) and V (dominant) degree chords can be used. Since we are in the key of G Major, G major is the tonic, C major is the subdominant and D is the dominant.
In the first measures Mozart plays the chords using arpeggios (the notes are played consecutively). He also uses chord inversions. When a chord is in inversion the lowest note is not the root. For example, in measure 2 Mozart uses a D dominant 7th chord (D - F# - A - C) but he uses the A as the lowest note (see Inversions).
In the next example we have:
Many melodies can be harmonized using only the tonic, dominant and subdominant chords because these chords have all the notes of the scale.