Harmonic Function
Harmonic Function is the description of how chords in a key behave. Studying a chord's function can help us define and describe its sound. This empowers us to identify chords by ear easier. There are three main kinds of harmonic function.
Tonic
The 1 chord serves as our tonic, or home, chord. A tonic chord brings ultimate resolution and is the most stable sound in a major key. Songs often begin and end on the 1 chord for this reason. Because of its stability, chord progressions usually make their way back to the 1 chord to properly resolve. Similarly, if a songwriter wants to leave a song unresolved, avoiding the 1 at the end can effectively do this.
Our ears will hear and assume a single chord or tone is 1 or tonic until given more musical context.
6m serves as a common substitute for 1 as a tonic chord. 1 and 6m share 2 notes so it's not surprising they share harmonic function. The minor is a nice, melancholy variation on our normal bright major tonic chord.
Dominant
The 5 chord serves as our main dominant chord. A dominant chord brings tension to a progression and has a great pull and desire to resolve to tonic. A quick look at chord progressions for simple songs shows us how common dominant resolving to tonic is.
The importance and prevalence of tonic-> dominant-> tonic harmonic motion can't be overstated. It developed in music history to be the most concise way to tell a harmonic story.
Tonic/Dominant motion can be likened to common movie plots:
protagonist (resolution, home, tonic)
struggle/antagonist/adversity (tension, dominant)
protagonist wins/overcomes adversity (back to home, resolution).
(The main reason why 5's pull to 1 is so strong is due to the 5 chord's 3rd, which is also the key's 7th scale degree. This 7th degree has a strong desire to resolve up to 1 by a 1/2 step (Ti -> Do).)
Another dominant chord is 7˚ (5 and 7˚ share two chord tones) but is so unstable due to it being diminished that its use in popular music is fairly uncommon.
Sub-Dominant or Pre-Dominant
Coming Soon!