An interval is a relation between two musical pitches or, in other words, the ratio of their frequencies. We can also define an interval as the distance between two notes. Named intervals are the following: unison, second, third, fourth, fifth, sixth, seventh, and octave. If tones appear together, they form a harmonic interval which refers to vertical connections. Musical chords consist of harmonic intervals. When two follow one another, the interval they form is melodic. In this case, it refers to horizontal connections. Musical scales consist of melodic intervals. When working with melodic intervals, we must remember that the interval is also defined relative to its lower pitch, as is the case with harmonic intervals.
The name “Octave” is borrowed from the Latin language and represents a literal translation: the eighth. It implies the eighth degree of the scale. Within the octave, we can form eight original intervals from tones of the diatonic and chromatic scales. Due to the alteration of its notes, the interval does not change. But the distance between both notes increases or decreases. That is why there is a division of interval quality into perfect, major, minor, diminished, and augmented. Any interval can be of diminished or augmented quality. Second, third, sixth, and seventh can also be major or minor. When unison, octave, fifth, and fourth are not diminished or augmented, they are perfect.