I’ve been asked this question many times and I’m convinced musicologists (including some that answered in the AM thread) make the answer harder than it needs to be.
Here is the stock answer I give that most people tend to respond to:
1) Think of the song from Sound of Music: “doe a deer, a female dear”. Got it in your head? (extra credit: try to get it OUT of your head!)
2) Next time you sit at a piano or guitar, pluck out any note and sing “do ray me fa so…” all the way up. You’re in the “key” of whatever note you originally plucked out.
3) Starting from any note skip ever other note (do-me-so OR ray-fa-la etc.). Those are chords.
4) When you play a bunch of chords the human brain wants these chords to “resolve” to the root chord of a key (the one starting from do). (This is because of an acoustical phenomenon in nature called “harmonics”.)
Music that is described as “tense” usually involves delaying the resolution to the root (like Thelonius Monk who rarely resolved). Music that feels “rooted” tends not to stray too far from the root at all (like John Lee Hooker who played one chord for 70+ years).
5) The chords in step 3 are called “triads” or three-part chords. Add a fourth note by skipping yet another one (do-me-so-ti OR ray-fa-la-do) and you have four-part chords or harmony. It’s when you get into four-part (like the Bach’s) that you really get a sense of wanting to resolve to the root.
Adding the 4th can really add a ton of color. For a C triad:
- “B” makes the chord “pretty”
- “B-flat” makes it “bluesy”
- “A” makes it Beatles
6) Since there are three or four (or more) notes in a chord but 12 notes in Western music that leaves a bunch of notes left over for the melody or solo you want to compose or improvise. When you have identified which non-chord notes sound best played or sung over the chord that’s called a “scale.”
For example for a 4-part C chord (C-E-G-B) you would think that “F” would be a good in-between note because it’s a C major scale. But it turns out F# is actually more interesting and pretty. In that case the scale is “G major.”
More on key vs. scale….
7) The term “key” is actually much, much looser than it used to be. It used to be tied to a specific “scale” (do, ray, me, fa… is actually a “major scale”). But starting with impressionistic classical and therefore jazz and blues, the notion that “key” and “scale” are the same just isn’t true any more. Since then the “key” is really only a hint at what scales are to be played or sung. What scale to play over what chords, or what chords to harmonize a given melody is high science and many classes in many musical institutions are devoted to it.
When jazz musicians talk of “key” they just mean “where to start.” I’ve never seen a key signature for “Giant Steps” and if someone says “Giant Steps in B” you better not be playing B major scale by the third beat (!!!) or you’re screwed.
The typical blues example uses the chords “C” and “F” and “G” and as long as you stick to triads you will never leave the “C” major scale. And you will never sound whiter.
The blues sound (“B flat” on top of the C triad) technically puts you into the “F” scale but that’s, you know, technically. In fact, you want to flat the 4th in ALL the chords in that progression to make it C7, F7 and G7. Now you end up playing/singing THREE different scales, one per chord in the course of the song. Everyone quickly agreed: It’s just “key of C” or “C blues.”
In the same way: If you take the “me” of the do-ray-me notes and make it a half step lower you are said to have the “minor” version of the “major” scale. It sounds more dark, melancholy or gyspy. It’s still “key of C” but the hint is amended by saying “key of C minor.”
8) A very common “mistake” a lot of non-music-nerds makes is to confuse inversions with different keys/chords. A C triad (C-E-G) is a C triad because it has those notes. Not because the bass player is playing C. If the bass player plays an E it’s still a C triad, it’s just now it has E in bass. (Sometimes written C/E or “C over E”) Piano players and guitarists are constantly telling me: “Here’s the next chord..” when really what they are doing is playing the same chord with a different bass note.
The order in which you play the notes from left to right on the piano is called the “voicing” or “arrangement” and is COMPLETELY separate from what note it is.
This is important:
I have found through years of arranging, composing, sampling and remixing that the ability to identify the true “root” note of a chord is much more valuable than trying to get the key of a whole song.
How do you that?
Well you can start by turning the treble down and trying hear what the bass player, cellist or left hand of the piano is playing because over 90% of the time they are playing the root.
Key is actually much, much less important than it used to be when trying to analyze the harmony quality of a piece of music. In music (especially post 1900) the two things that matter the most is the actual chord progression (which chords in which order) followed closely by voicings and arrangement (the layout of the chords).