When developing a highly trained ear, you need to have a deep understanding of two things:
Intervals
Scale Degrees
Both are very similar but mean two different things. Most guitar players get it wrong when understanding the difference between the two. So, let's start by defining both.
Intervals
An interval is the distance between two notes, which is measured in whole steps and half steps. Intervals can be classified as major, minor, perfect, augmented, or diminished, each referring to a specific distance between notes. For example, a major third interval signifies a larger pitch gap than a minor third.
Scale Degrees
On the other hand, a scale degree refers to which note of the scale is being played. It is always referring to the one of the key. That is the main difference.
An interval can judge the distance from any two notes in the key, while a single degree will always judge the distance from the key's root. For example, if we are in the key of C, we would have notes:
C D E F G A B
C is the 1st scale degree, D is the 2nd scale degree, E is the 3rd scale degree, and so on.
If I were to play a D and then an A note, they would be a 5th apart. But the D is the 2nd scale degree, and A is the 6th.
This is important to ear training because when The C chord is being played, we will hear all the scale degrees in relation to C. But when any other chord is played, we will hear the intervals based on the chord's root.
You can get an even more in-depth description of this below.
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