This lesson focuses on the various ways intervals are classified. It's not as important as the previous sections, but is included for completeness. It will be more useful as a reference rather than as information to be studied and memorised.
SIMPLE AND COMPOUND INTERVALS.
If an interval is smaller than an
octave, it is a
simple interval.
If it is an
octave or greater, then it's classed as a
compound interval.
So far, the highest interval number that we've mentioned is
8 (the
OCTAVE) but you will often come across three more interval numbers, namely:
9,
11 and
13, mostly in chord names. They are known as
compound intervals and are just
simple intervals which have been expanded by an octave. There is little difference in effect between a simple and compound interval. Mostly it's a naming convention used with chords.
For example,
C to
D is a
SECOND, (2 letters,
C &
D); but
C to the next higher
D is a
ninth (9 letters,
C D E F G A B C &
D).
To convert a compound interval back to its simple form, subtract
SEVEN.
A
NINTH is really a
SECOND.
An
ELEVENTH is really a
FOURTH.
A
THIRTEENTH is really a
SIXTH.
INVERTED INTERVALS
If we reverse (or
invert) the order of notes of a simple interval, the interval changes number and quality.
For example
C to
E can be inverted as
E to
C.
C to
E is a
THIRD (3 letters -
C, D &
E).
E to
C is a
SIXTH (6 letters -
E F G A B &
C).
To invert an interval, just subtract its number from
9.
SECONDS become
SEVENTHS.
THIRDS become
SIXTHS.
FOURTHS become
FIFTHS.
The interval quality also changes, as follows:
MAJOR intervals become
MINOR.
MINOR intervals become
MAJOR.
AUGMENTED intervals become
DIMINISHED.
DIMINISHED intervals become
AUGMENTED.
PERFECT intervals remain
PERFECT.
Examples
A to
C is a
MINOR THIRD.
C to
A is a
MAJOR SIXTH.
E to
F is a
MINOR SECOND.
F to
E is a
MAJOR SEVENTH.
A to
E is a
PERFECT FIFTH.
E to
A is a
PERFECT FOURTH.
C to
F# is an
AUGMENTED FOURTH.
F# to
C is a
DIMINISHED FIFTH.
ENHARMONIC INTERVALS
In many cases two intervals will sound exactly the same yet they have different names.
For example
C to
D# is an
augmented second but
C to
Eb is a
minor third.
D# and
Eb are the same sound so the intervals containing them will sound the same, but will have different names. Such intervals are known as
ENHARMONIC EQUIVALENTS of each other.
Examples
C to
F# (
augmented fourth) sounds the same as
C to
Gb (
diminished fifth).
D to
A# (
augmented fifth) sounds the same as
D to
Bb (
minor sixth).
HARMONIC AND MELODIC INTERVALS
When the two notes of an interval are heard at the same time, the interval is said to be
HARMONIC.
When the two notes are heard one after the other, the interval is said to be
MELODIC.
Melodic intervals (apart from unisons) may be further described as
ascending or
descending, depending on the pitch order of the notes. Remember, though, that intervals are
always measured from the lower note to the higher note, regardless of whether the notes are ascending or descending.
CONSONANT AND DISSONANT INTERVALS
Consonance and
dissonance are terms used to describe how well the notes of an interval blend together.
Consonance is the quality we hear when the two notes combine well and produce a sound which, at best, is pleasant and agreeable and, at worst, bland and colourless.
consonant intervals include all the
perfect intervals, i.e.
unisons, octaves, fourths and
fifths. Those are classed as
perfect consonances.
They also include
major and
minor thirds as well as
major and minor sixths. These are classed as
imperfect consonances. They still combine well but not as well as the perfect intervals. This slight mismatch gives them an
edge that makes them sound far more interesting than than the pure consonances. This is the reason that chords are built from thirds.
Dissonance is the opposite of consonance. An interval is dissonant if its notes seem to clash when they combine. If played without care the effect will usually be harsh and unpleasant. If played with care in a well composed setting, the effect may be a thrilling build up of psychological tension in the listener that seeks to find emotional release in well-chosen consonant intervals. This is basically how most western harmony works.
Dissonant intervals include
major and minor seconds,
major and minor sevenths and the
augmented fourth or its enharmonic equivalent,
the diminished fifth.
There are also certain circumstances in which the
perfect fourth (officially a perfect consonance) can sound dissonant, (e.g. when heard within a
suspended fourth chord)
Examples
C to
G is a
perfect fifth and highly
consonant. Being so consonant means it has very little harmonic effect when heard within chords.
C to
Gb is a
diminished fifth and is highly
dissonant. This interval is also called a
tritone because it consists of
three whole tones or
whole steps. When first discovered in early medieval times, its pungent dissonance led to its avoidance by many composers. It gradually became, and remains, the most important dissonant interval in western music.