When two notes played simultaneously are separated by an octave, our brain struggles to differentiate them. That's why we give both sounds the same name. In music, the octave is the purest example of consonance. All other intervals tend to deviate from this original stability, gradually introducing the notion of instability or dissonance.
Learn to classify musical intervals based on their stability. In this exercise, the emphasis is placed on the octave, 5th, and 4th, which are considered the most stable (perfect) intervals, and on the Major 7th and minor 2nd, which are considered the most unstable intervals.
Remember: unstable intervals have a stronger tendency to dynamically resolve towards a stable interval.
For example, in the case of the Major 7th C-B, which is considered highly unstable, the note B is drawn towards the octave C. This instability tends to resolve towards the octave C-C, which is perceived as more stable.
