You can identify an interval as simple as that the distance between two notes. Most concepts in music theory are expressed in terms of intervals so the better you know this terminology, the better your understanding of music theory will be.
There are two basic types of intervals;
An interval less than octave. The most common simple intervals are unison, minor second, major second, minor third, major third, perfect fourth, tritone, perfect fifth, minor sixth, minor seventh, major seventh, but also include any interval below an eighth, or the octave.
An interval greater than an octave, such as ninth, tenth, etc... But intervals larger than a thirteenth are rarely spoken of.
|8ve plus 2nd (octave plus second)
|8ve plus 3rd (octave plus third)
|8ve plus 4th (octave plus fourth)
|8ve plus 5th (octave plus fifth)
|8ve plus 6th (octave plus sixth)
|8ve plus 7th (octave plus seventh)
As you can easly realize that if you subtract 7 from a compound interval you can find the related common interval and vice versa.
Interval Number And Quality
The tonic is the first note of a musical scale, and in the tonal method of music composition it is extremely important in musical notation. On the other hand the staff or stave is a set of five horizontal lines on which note symbols are placed to indicate pitch and rhythm. The lines are numbered from bottom to top; the bottom line is the first line and the top line is the fifth line. The interval number of a note from a given tonic note is the number of staff positions enclosed within the interval
The name of any interval is further qualified using the terms perfect, major, minor, augmented, and diminished. This is called its interval quality.
Unison, fourth, fifth, octave. These intervals may be perfect, augmented, or diminished.
A perfect fourth is five semitones.
A perfect fifth is seven semitones.
A perfect octave is twelve semitones.
A perfect unison occurs between notes of the same pitch, so it is zero semitones.
In each case, an augmented interval contains one more semitone, a diminished interval one fewer.
Second, third, sixth, seventh. These intervals may be major, minor, augmented, or diminished.
Major seconds are two semitones, also called a whole step,
Minor seconds are one semitone, also called a half step.
Major thirds are four semitones.
Minor thirds are three semitones.
Major sixths are nine semitones.
Minor sixths are eight semitones.
Major sevenths are eleven semitones.
Minor sevenths are ten semitones.
In each case, the augmented interval contains one semitone more than the major interval, and the diminished interval one semitone fewer than the minor interval.
Intervals (part 2)