The most common related chords are those built on the fourth and fifth notes
of the scale.
For simplicity, these chords are often referred to just by their number—usually
with Roman numerals such as I, IV, and V. The V chord is G major in this case, and
the G7 is formed by adding the seventh note above the G.
Here, (a) shows the necessary components of the G chord: G, B, and D.
The seventh note above G is F, and adding this as in (b) makes a familiar G7.
The V chord is known as the dominant, and when the seventh is added it is known
as the dominant seventh.
The IV chord, F in this case, is also known as the subdominant. There are hundreds
of songs that only use the I, IV, and V chords, and understanding this makes it
easy to change key.
For instance, if you are in the key of C and the song feels too low for your
voice, you can try D a tone higher. The V chord in D, counting up the scale, is
A. A7 will be the dominant seventh. The fourth note of the D scale is G, so G will
be the IV chord or subdominant. So everywhere you had C as I chord, you may substitute
a D chord, and similarly you may replace the G7 chords with A7s, and the Fs with