A perfect fourth is an interval of 5 semitones (half steps) between 2 notes. For example, if you play middle C on the piano followed by the F above it then you will have played a perfect 4th interval:
The F is 5 semitones higher than the C and so the interval is described as a perfect fourth.
This can be repeated on any starting note of the chromatic scale. For example, if you play a G sharp on the piano and count up 5 semitones you will reach C sharp – this interval is also a perfect fourth.
Here is a summary of perfect 4th intervals starting on each of the notes of the chromatic scale:
Note how each of the letter names are 4 spaces apart (e.g. C-D-E-F) – hence, why it is called a “fourth”. You can also see that the notes will also usually be either naturals, flats or sharps except for F-Bb and F#-B.
Examples of Perfect Fourths
There are lots of famous examples of melodies that use the perfect 4th interval as the start of the song/piece of music. If you can remember one or two of these then it will really help you to be able to recognise the interval when you hear it.
The opening to the well known Christmas carol Hark the herald Angels Sing starts with a perfect 4th:
The start of Away in a Manger and O Little Town of Bethlehem are other examples of Christmas carols that use the same interval in the opening 2 notes of the melody.
Also, the opening 2 notes of Amazing Grace are another great example to remember:
Other examples of vocal music you could listen to are the opening of the Hallelujah chorus from Handel’s Messiah which has a descending interval and the start of Auld Lang Syne.
If you are looking for an example in instrumental music then the opening of Beethoven’s famous Moonlight sonata slow movement uses a repeated first inversion chord pattern where the opening 2 notes form the interval of a perfect fourth:
Harmonic Intervals
There are a couple of common harmonic situations in which perfect 4ths occur – it is good to be aware of these and have an understanding of them.
The first is that perfect 4ths are found in major and minor chords when they are played in their 1st and 2nd inversions.
For example, a C major chord played in its root position contains the notes C-E-G. The interval between the C and the E is a major 3rd and the interval between the E and the G is a minor 3rd.
However, if we play the chord in its 1st inversion (the E is now at the bottom of the chord) then the interval between the E and the G is still a minor 3rd, but the interval between the G and the C is a perfect fourth.
If we now play the chord in its 2nd inversion with the G at the bottom the intervals become G-C (perfect 4th) ad C-E (major 3rd).
You can see how the perfect fourth is a crucial element of these chords when played in their 1st and 2nd inversions. It also occurs in the minor chord inversions as well.
The perfect 4th interval is also a key element of the 4-3 suspension – you can see the lesson on suspensions if you would like to find out more about their use in this context.
