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27: Tonic Function

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Lesson 27: Tonic Function

This lesson starts exploring the different effects of the different major-key diatonic triads. Before you take this lesson, you should know:
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:
  • VIm
  • IIIm

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 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 28: Subdominant & Dominant.

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1: Introduction
2: Teaching Yourself
3: Pitch & Keyboard
4: Pitch Names
5: Letters Game
6: Sharps & Flats
7: Half-Steps
8: Whole-Steps
9: Steps Game
10: Scales
11: Major Scale 1-2-3
12: Major 1-2-3 Games
13: Major Scale 1-5
14: Major 1-5 Games
15: Chords: Major Triads
16: Major Triad Games
17: Minor Triads
18: Minor Triad Games
19: Major Scale 1-8
20: Major Scale Games
21: Scales Above 8
22: What Next
23: Keys
24: Roman Numeral Chords
25: Diatonic Triads
26: Using Diatonics
27: Tonic Function
28: Subdominant & Dominant
29: Diatonic Function Analysis
30: 7th Chords
31: 7ths Games
32: Treble Staff
33: Treble Staff Game
34: Melody: Chord Tones
35: Pitch & Frequency

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