Lesson 24: Tonic Function
This lesson teaches the
different effects of the different
major-key
diatonic triads you learned about in
Lesson 22: Diatonic Triads.
To review, the
common major-key diatonic triads are:
I IIm IIIm IV V VIm
This lesson explores the
tonic function chords:
I,
IIIm,
and
VIm.
The I Chord
Let's start with the
I chord ("the One chord"), also called the
tonic chord. ("Tonic" is another name for the 1st note, or "degree 1",
of a scale; that's where the term
tonic chord comes from.) The tonic
chord is the
home chord in a song's key.
When the tonic chord is
playing, it feels like you're "at home" or "at rest";
when any other
chord is playing, it feels like you're
not "at home" (with some
partial exceptions we'll get to below).
Musical phrases
often start or end (or both) on the tonic chord; if a
phrase
doesn't do this, then the whole phrase tends to feel "not at
home", and it's likely to be preceded and/or followed by phrases which
do start or end on the tonic chord.
The VIm and IIIm Chords
The I chord is "the" tonic chord, but two other diatonic triads also have
tonic function; that is, they have an "at home" effect similar to (but
weaker than) the I chord. These "
substitute tonic" chords are:
Why do the VIm and IIIm chords have a "tonic" effect like the I chord?
Basically, because they share most of the same pitches. For example, in the
key of C:
| I | = C | = (c e g) |
| VIm | = Am | = (a c e) |
| IIIm | = Em | = (e g b) |
The VIm chord. Of these two substitute tonic chords,
VIm is
probably "stronger", for a couple of reasons:
-
It contains the "tonic pitch" of the song's key (e.g., in the example above,
the Am chord contains the pitch "c", which is the tonic pitch for the
key of C).
-
The VIm chord is also the Im chord (the tonic chord) of the
relative minor key to the major key we're in. In the example above,
the Am chord is the tonic chord in the key of A minor; the key
of A minor is the relative minor key to the key of C major. (See
Lesson 27: Natural Minor Scale,
which leads to relative minor keys.) In fact, you can make a major-key song
temporarily sound like it's in its relative minor key, just by using
the VIm chord (instead of the I chord) as the "home chord" in that section
of the song. This is actually a very commonly-used effect.
The IIIm chord has a "weaker" or more "passive" tonic effect, probably
because it
doesn't contain the tonic pitch (in the example above, the
Em chord does
not contain the pitch "c"). I usually see the IIIm
chord used in a song for one of these reasons:
Mixing Tonic and Non-Tonic Chords
The "dance" between tonic and non-tonic chords in a song is a fundamental
aspect of the song's musical effect. If a song
stays on only tonic
chords for a long time, the effect is "static" or "monotonous". If a song
stays away from tonic chords for too long, the effect is "wandering" or
"lost".
This "chord function dance" is just one of many techniques for using an even
more universal principle in music:
tension and resolution.
Tension is when the music feels "unsettled" (e.g., non-tonic chords);
resolution is when the music feels "settled" (e.g., tonic chords).
Moving from tension to resolution is one of the basic principles for creating
musical pleasure.
Next, go on and learn about the
other diatonic chord functions in
Lesson 25: Subdominant & Dominant.
