INTERVAL NUMBER
A more complete and systematic way to name intervals 'of any size' is by numbering them according to how many scale degrees (letter names) are covered by the interval. This is easy. To find the number of any interval, you just count
UP the letters. (Intervals are always measured from low to high.)
For example, the notes
A to
B form the interval of a
SECOND, because the interval between them spans two letters:
A &
B. Numbering intervals is as simple as that.
Examples
D to
G is a
FOURTH because there are four letters involved
D,
E, F, &
G.
F to
A is a
THIRD because there are three letters involved
F,
G &
A.
C to
C (both notes exactly the same pitch) is a
FIRST because only one letter is involved:
C. But we call this interval a
UNISON instead of a '
first' (from
UNI = 1).
C to the next
C higher up is an
EIGHTH because there are 8 letters involved (
C D E F G A B &
C). But we call it an
OCTAVE instead of an '
eighth (from
OCTA = 8).
Don't worry about
sharps or
flats when numbering intervals. They make no difference to how an interval is numbered.
The following intervals are ALL
thirds because only three letters are involved (
A B &
C):
A to
C
A to
C#
Ab to
C
Ab to
C#.
INTERVAL QUALITY
As we saw in the last section, the intervals
A to
C and
A to
C# are both
thirds because they both span three letters
A B &
C. But clearly, they're not the same.
A to
C# is larger than
A to
C, (by one fret or
semitone) so it's necessary to qualify them so that we can distinguish between them. That's done by using the words
MAJOR to mean the larger one, and
MINOR to mean the smaller one.
So, to give those intervals their full name:
A to
C is a
MINOR THIRD
A to
C# is a
MAJOR THIRD.
If you want to count the semitones between them, you can see that
MINOR THIRDS are 3
semitones (3 half steps)
MAJOR THIRDS are 4
semitones (4 half steps)
We can add the words
MAJOR and
MINOR not only to
THIRDS but also to
SECONDS,
SIXTHS and
SEVENTHS.
C to
D is a
MAJOR SECOND (2
semitones)
C to
Db is a
MINOR SECOND (1
semitone)
C to
A is a
MAJOR SIXTH (9
semitones)
C to
Ab is a
MINOR SIXTH (8
semitones)
C to
B is a
MAJOR SEVENTH (11
semitones)
C to
Bb is a
MINOR SEVENTH (10
semitones)
Perfect intervals
Some intervals are called
perfect. They sound so pure when we hear them that it's tempting to think that that's why they're called 'perfect'. In fact, they're called
perfect for historical reasons that we needn't bother with in this lesson.
The perfect intervals are the
unison,
octave,
fifth and
fourth.
Augmented and diminished intervals
If we stretch a
major or a
perfect interval by
one semitone (but keeping the same letters), the interval number stays the same but we call it
augmented instead of
major or
perfect.
Examples
C to
F is a
PERFECT FOURTH (5
semitones)
C to
F# is an
AUGMENTED FOURTH (6
semitones)
C to
D is a
MAJOR SECOND (2
semitones)
C to
D# is an
AUGMENTED SECOND (3
semitones)
If we shrink a
minor or
perfect interval by
one semitone, it becomes
DIMINISHED.
Examples
B to
A is a
MINOR SEVENTH (10
semitones)
B to
Ab is a
DIMINISHED SEVENTH (9
semitones)
B to
F# is a
PERFECT FIFTH (7
semitones)
B to
F is a
DIMINISHED FIFTH (6
semitones)