Perfect 4ths

What is the interval of a perfect 4th?

A perfect 4th interval consists of two notes five half steps apart. Intervals tell us the distance between two pitches, so a perfect 4th consists of two notes five half steps apart. Other types of fourths are the augmented fourth, or a tritone which is 6 half steps apart, or the diminished fourth, which is four half steps apart. 

You can learn more about intervals in our article on Intervals in Music Theory.

Why is it called a "perfect" 4th?

The “perfect” intervals are unisons, fourths, fifths, and octaves. These intervals each have a harmonic series that meshes well together. The perfect 4th is an inversion of the perfect 5th, which is another interval that lines up in this harmonic sequence. Mathematically, this means that perfect intervals are related by ratios where the harmonic series lines up. It sounds open or pleasant to our ears. This alignment also means that perfect intervals will never be major or minor. 

How many half steps are in a perfect 4th?

A perfect 4th has five half steps. Count five half steps up or down to create this interval. You can practice building this interval using our Perfect 4ths Theory Worksheet which is included with Premium. Remember to double check your key signatures as you work to determine what kind of fourth you are looking at! Sometimes in sheet music, two notes can look like they are related by a perfect fourth, but this can be deceiving. As an example, the interval from F natural to B natural looks like a fourth, but because it covers 6 half steps, it’s actually an augmented fourth, or tritone. 

Learn all about musical intervals–including their names, how to write them on the staff, and how to find them on the keyboard–with our free Guide to All Intervals. 

How to identify a perfect fourth by ear

One way to practice identifying perfect fourths by ear is to find a song that uses that interval and practice that melodic step or skip. A great example of an ascending perfect fourth is the beginning of the Christmas carol “Hark! The Herald Angels Sing” or the start of the song “Auld Lang Syne.” For a descending example, you can think of the second and third words of “O Come All Ye Faithful.” The interval between “Come” and “All” is a descending perfect fourth. In all of these examples, the melody moves from SO up to the next DO, or from DO down to the next SO in solfege.

What is the ratio of a perfect fourth?

In just tuning, the ratio of a perfect fourth is a 4:3 ratio. 

What is the sound of a perfect fourth?

This harmonic interval often sounds open and pure. This quality stems from its consonance with the harmonic series above each of the two notes. Melodically, an ascending perfect fourth can be exciting, as in the start of the Star Wars theme or the tune “Here Comes the Bride.”  

What are common uses for a perfect 4th in music?

In music, you’ll often hear perfect 4ths at the beginning of melodies that move from “SO” to “DO.” This is a common way for composers to start a song. It happens any time a major chord is played in first inversion as well. You’ll also hear perfect fourths in suspensions. In a sus4 chord, the fourth creates tension by replacing the third of the chord before resolving downward to that third. These intervals are also a major component of quartal harmony, where modern composers stack perfect fourths instead of the more traditional thirds.