After triads, the next most common chord type is the seventh chord, (sometimes called the dominant seventh chord). Seventh chords consist of four notes taken from the major scale of the same letter name. These notes are the first (1), third (3), fifth (5) and flattened seventh (b7) notes of the major scale, so the chord formula for the seventh chord is:
A flattened seventh (b7) is created by lowering the seventh note of the major scale by one semitone. This is the same b7 note that is found in the Blues scale. Notice that the seventh chord is simply a major chord with a flattened seventh note added.
The G7 chord can be constructed from the G major scale. Using the seventh chord formula on the G major scale gives the notes G, B, D and F. When the seventh note of the G major scale (F#) is flattened, it becomes an F natural.