Quantity & Quality of Musical Intervals

Without intervals we wouldn’t have melody, chords, or scales.  They really are one of the foundations of music.  An interval in music is defined as a distance in pitch between any two notes.  The larger the interval between two notes, then the greater the difference in pitch between the notes.  And vice versa, the smaller the interval between two notes then the smaller the pitch between the notes.  Semitones and tones (or half steps and whole steps) are the building blocks of intervals.  We use different combinations of them to make up all the different types of scales and chords that make music sound so different.  The smallest possible interval (in western music) is a semitone.  A semitone is the very next higher or lower note. 

For example, from E to F or from C to C sharp (C#) on a piano keyboard. 

Quantity and Quality of Musical Intervals -1
A tone is an interval of two semitones. 
Quantity & Quality of Musical Intervals -2
We describe intervals using numbers depending on how many letter names of the musical alphabet there are between the two notes.  
For example, the notes C and D are two letter notes apart and so is an interval of a 2nd. 
Quantity & Quality of Musical Intervals -3
But if we look at the notes C and E they are three letter notes apart and so this interval is a 3rd. 
The notes C to F are four letter notes apart and so would be a 4th.  
This just carries on, C to G is five letter notes and so would be a 5th. 
C to A is six letter notes and so would be a 6th. 
C to B is seven letter notes and so would be a 7th. 
Lastly, C to the C above it is eight letter notes and so is an interval of an 8th or octave. 
Unison intervals are the same note. 
For example, two different instruments might play exactly the same note in a piece of music. 

Interval quality. 

I’ll explain why we need to distinguish the interval quality with the example below. 
Both of these intervals are a third: 
In the first example, C to Eb is three letter notes: C – D – Eb and so is a 3rd. And in the second example, C to E natural is also three letter notes: C – D – E and so is a third.  We need to distinguish the type of interval (quality) to differentiate between them both. 
There are five different types of quality of interval which are: 
• perfect intervals 
• major intervals 
• augmented intervals 
• minor intervals 
• diminished intervals 

 Perfect intervals. 
There are three intervals that are what we call perfect intervals: 
 • a perfect 4th 
• a perfect 5th 
• a perfect 8ve (or octave) 
To be a perfect interval the upper note has to be in the major scale of the lower note.  If the interval is a 4th, 5th or 8ve and isn’t in the major scale, then it’s not a perfect interval.  For example C to F# is a 4th but is not a perfect 4th as F# is not in C major scale. 
Below are all the perfect intervals: 
Next we’ll look at the other intervals in a major scale which are major intervals. 

Major intervals. 
There are four intervals that are called major intervals: 
• a major 2nd 
• a major 3rd 
• a major 6th 
• a major 7th 
So if the upper note of an interval is in the major scale of the lower note (and it’s not a 4th, 5th or 8ve) then it will be a major interval.  When answering questions about intervals you should always work out the number of the interval first by using the lower note as number one and counting how many letter notes there are to the higher note.  Then, if the upper note is in the major scale of the lower note you know that it is going to be either a major interval or a perfect interval. 
Here’s C major scale with the major intervals marked:   
This is the case for every major scale, not just C major. 

Minor intervals. 
If we take any of the major intervals we looked at above and make them smaller by one semitone (half step) then they now are minor intervals.  For example, if we took C to E which is a major 3rd and flattened the E to make it an Eb, it now becomes a minor 3rd.   
Because there are only four major intervals there are also only four minor intervals possible which are: 
• minor 2nds 
• minor 3rds 
• minor 6ths 
• minor 7ths 
Here is F major scale but with the 2nd, 3rd, 6ths and 7th notes flattened to become minor intervals. 
So to recap: 
• If the upper note is in the major scale of the lower note it will be either a major or perfect interval 
• If it’s in the major scale and it’s a 4th, 5th or an 8ve then it will be a perfect interval 
• If it’s in the major scale and is a 2nd, 3rd, 6th or 7th then it will be a major interval 
• If it’s a semitone lower than a major interval then it will be a minor interval 

Augmented Intervals. 
An interval becomes augmented when we extend a major or perfect interval by one semitone (half step) without changing the letter name.  So if we took a major second like F to G and made the G a G#, then we’ve made the interval wider by one semitone and so it’s now an augmented 2nd. 
It’s the same with perfect intervals, for example, the interval F to C is a perfect 5th but if we make it F to C# then it’s now an augmented 5th. 
So here is F major scale again but with all the notes raised by one semitone and they’re now all augmented intervals.   

Diminished intervals. 
If we flatten any of the three perfect intervals – 4ths, 5ths or 8ves they don’t become minor, they become diminished intervals. Let’s take the two notes A and D which is a perfect 4th. 
If we were to flatten the D to make it a Db it would now become a diminished 4th. 
We can flatten a minor interval by another semitone and it also becomes diminished. For example, E to D is a minor 7th, but if we make the D one semitone lower to a Db it then becomes a diminished 7th. 
So just remember that: 
• if we flatten a perfect interval by one semitone (half step) it becomes a diminished interval 
• If we flatten a minor interval by a semitone (half step) it becomes a diminished interval 
• If we flatten a major interval by a tone (whole step) it becomes a diminished interval 

Harmonic and melodic intervals. 

As well as categorising intervals into their interval numbers: 2nds 3rds 6ths etc, and by the interval quality: major minor perfect etc, we can also categorise intervals into two other groups: 
• Harmonic intervals 
• Melodic intervals 
Harmonic intervals are how we describe two notes that are played at the same time.  They are played in harmony and so are a harmonic interval. 
The opposite of a harmonic interval is a melodic interval which is where the two notes are played one after the other.  They are part of a melody and so are a melodic interval.     

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