24: Tonic Function
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Lesson 24: Tonic FunctionThis 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:
The VIm chord. Of these two substitute tonic chords, VIm is probably "stronger", for a couple of reasons:
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.