Key

A chord is "in-key" if its chord tones are composed of notes that exist in the key you're playing in.

To derive the chords in the C major scale, we follow this process:

for each scale degree, build off the 1-3-5.
if the 1-3-5 notes all exist in the C major scale, then the chord is good
if the note does not exist in the C major scale, then it needs to be flattened
- ex. building 1-3-5 off the second scale degree of C major is D-G♭-A. Since G♭ is not in the key of C major, it needs to be flattened to F. Therefore, the second chord of C major is D minor.

Think of the notes of a key as different pieces of punctuation. The root is like the period, since it brings some sense of finality. The 4th is like a comma, which implies there is more to come.

The 3rd is the most melodic part of the key. To make phrases sound more melodic, give more prominence to the 3rd.

The chords of a given key will all have the notes of that scale

ex. In the key of C major, we have C, Dm, Em, F, G, Am, Bdim. Every note that makes up each of these chords can be found in the C major scale.
if we’re playing other notes along with the triads, they will sound “right” as long as they are in the key. However, it’s critical to observe the note hierarchy. If we’re adding any of the chord tones (is. 1-3-5) to the triad, it will sound smooth. But if we add a 4th, it may clash, depending on if the chord has an E in it (in C major, E is the 3rd, and the 3 clashes with the 4 due to them being only a semitone apart).

How to know which notes to play

Knowing which notes to play is not so much matter of which key we're in, but more which chord is currently playing. The key gives us the general framework to know which chords could be played, but our general goal is to play notes that fit well over the chord (note: dissonance is used as a desirable part of music, so whether or not it "fits" is determined by how proper it sounds)

Generally speaking, a note fits well over a chord if it is either a chord tone or is more than a semitone away from one of the chord tones. In major, this would be the 1, 2, 3, 5, and 6. And in minor, this would be the 1, ♭3, 4, 5, ♭7. We'll notice that these scale degrees make up the major and minor pentatonic scales.

If we were to take each of the 7 notes of the major scale, what we would find is that we can build a pentatonic scale off of that note, and each of those notes would fall neatly into the tonic's series of notes.

expl: taking C major since it has no accidentals, a major pentatonic build on d, e, f, g, a, b or c will all contain the notes a, b, c, d, e, f, g
the logic follows that any pentatonic note of the chord that is currently playing will sound good
pentatonic of course is not the whole story. Over a major chord, we can also play the 4 and 7, and over minor the 2 and 6. However, what we'll notice is that unlike pentatonic, some or all of these notes may not sound good. If we are in the key of C and wanted to play a G major chord in our progression, we would find that the 4th of the G major scale sounds good, but the 7th does not. This is because the 7th of the G major scale is a F#, which is not in the key of C major. To "fix" it, we have to remove the sharp from the F. The resulting scale is 1-2-3-4-5-6-♭7-1; Mixolydian.

Key vs. Tonic: An Important Distinction

The key and the tonic are related but distinct concepts, and understanding the difference is crucial for analyzing and playing music effectively.

Key

The framework of notes and chords available to you

Identifies which collection of notes the song draws from
Tells you the "family" of chords that will generally work
Provides a reference system for naming chord relationships (I, ii, iii, IV, V, etc.)
Can be identified through analytical clues in the progression

Tonic

The chord that feels like "home" or resolution

The emotional/harmonic center of gravity
Where the song feels at rest
May or may not be the I chord of the key
Determined by listening, not just analysis

Example: "Good Times" by Chic

The progression is Dm → G (repeating). Key: C major

We know this because the only time you move up a 4th from a minor chord and end up on a major chord is ii-V

This framework tells us other chords like Em, Am, Cmaj7, etc. would fit

Tonic: D minor

Dm is where the song feels resolved and grounded
The song never plays C major
The song is actually functioning in D Dorian mode

Why Both Matter:

Identifying the key gives you analytical tools and helps you spot patterns. For instance, when you hear two major chords a whole step apart (like C and D major), you've identified IV and V, which tells you the key is G - even if G never appears as the tonic.
Understanding the tonic tells you how the music actually feels and functions, guiding your phrasing, improvisation, and emotional interpretation.
Bottom line: The key is your map; the tonic is your destination. They're often the same, but when they're not, recognizing the difference unlocks a deeper understanding of how the music works.

Relative key

Two keys are relative if they contain all of the same notes

ex. C and Am

The relative minor is 3 semitones below the tonic of the major scale

ex. 3 semitones below the C is A, meaning that the relative minor of C is Am

Modulation

Also known as key change

Modulation to the subdominant key often creates a sense of musical relaxation, as opposed to modulation to the dominant (fifth note of the scale), which increases tension.

ex. If we are playing in C major, modulation to the subdominant means a key change to F

Going to new keys can be done by preceding the new tonic with its dominant chord. This works because dominant chords want us to move to the tonic.

ex. If we’re moving to the key of A, we can play an E immediately before to softly land us into the new key.

Just because a section starts in vi doesn’t mean we have modulated to minor.

ex. Consider Drive My Car, which starts in major but centers around the vi at the chorus. Despite this, it doesn’t really have a minor feel. Compare this to We Can Work It Out, which is in major but starts on the vi during the chorus. This section on the other hand does have a minor feel. This is in part due to the V-i movement that reinforces the minor feel

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