The importance of key in understanding and creating musicI was at a jam session the other night. A couple of other musicians started playing an unfamiliar song. I listened to it for a little while and then started joining in with the chords at the keyboard.
How did I do that, since I don't have perfect pitch? Well, first of all I had to work out what key they were in, so I tried a few notes on the piano until I found the one that matched the keynote. Then I just listened to the chords. Irrespective of what key I'm in, I can recognize the tonic chord, dominant chord, subdominant chord, chord of the relative minor and all the other common ones. I then used my theoretical knowledge to translate those chords into the relevant key. So if we'd been in D major and I heard tonic-dominant-subdominant-dominant, I'd have played D major-A major-G major-A major. I wouldn't normally have to think about that sort of thing consciously, unless we were in a remote key with lots of sharps or flats, or unless the piece used lots of chords remote from the actual key.
It's the same when I'm composing music. In my head I imagine a tonic chord, or a dominant seventh, or whatever. When I come to actually play the piece on the keyboard, or write it down, I'll choose an appropriate key and use the appropriate chord sequence. Ditto the melody - it doesn't really matter what key I imagine the melody in, as long as I sing it in the same key as I'm playing in.
So you can see that, for me at least, the key is literally the "key" to understanding the whole song. If I had no concept of key, I'd be completely lost. I don't really think of chords as C major or G major when I'm composing. The C major chord has no special significance of its own, except relative to a given key. If I'm in C major, it's the tonic chord - the "home" chord that the song finishes on. But if I'm in F major, it's the dominant chord - the one that naturally leads towards the tonic chord and requires resolution. It has a very different character.
Let's use Ralph McTell's "Streets of London" as an example. I won't write out all the chords - just the first and last ones of each pair of lines. It's in C major.
C [Have you seen the old man, in the closed-down market/Picking up the papers in his worn-out shoes] G7
C [In his eyes you see no pride, hand held loosely by his side/Yesterday's papers, telling yesterday's news] C
F [So how can you tell me, you're lo - ne - ly/And say for you that the sun don't shine?] G7
C [Let me take you by the hand, and lead you through the streets of London/I'll show you something, to make you change your mind] C
The first pair of lines starts with C major, the tonic or "home" chord, as is common in many songs. It ends with G major (ignore the seventh for now), the dominant chord, creating an "unresolved" feeling. You couldn't finish the song there.
Then we go back to the tonic chord for the start of the next pair of lines, and we finish on the tonic chord at the end of the verse. The song could, in theory, end there, although it'd be pretty short! There's a feeling that we've arrived back home again.
The chorus starts on F major, the subdominant chord, which is quite common, creating a change of key-colour. We go through a brief key-change in the middle of the line before arriving at the dominant chord again, G major. We feel as though we're heading towards a conclusion. The final section begins and ends on the tonic chord, and there's a resolution to the song.
Now let's transpose the song into F major.
F ------------------------- C7
F ------------------------- F
Bb ----------------------- C7
F ------------------------- F
The chord of C major (ignore the seventh again) is now playing the role that G major did in the earlier key - the dominant chord. It's the one that requires resolution. We don't feel we've completed the song until we've got back onto F major.
So my point is that it's not the actual letter-names of the chords that are important. It's their roles relative to the key you're playing in.
When you're singing, unless you've got perfect pitch, you've got no absolute concept of pitch - it's all relative. In any given key, you've hopefully got a feel for which note is the keynote, which is the fifth, which is the third and so on, but not for the actual letter-names of the notes. That's why it's so easy to sing a song in several different keys (if your range allows it). You don't have to think about transposing as you do with an instrument - it happens automatically. That's why some people recommend tonic sol-fa as a way of teaching singing (doh-ray-me). "Doh" is always the keynote of the scale, regardless of what key you're in. "Soh" is always the fifth, "me" is always the third, and so on.
That's not true with most instruments, of course. Nevertheless it's the way I think when I'm playing by ear, or when I'm composing in my head. The realization of "doh-ray-me" at the keyboard might be "C-D-E" or "F-G-A" but really that's irrelevant. It's the relative pitches of the notes that are important, not the absolute ones.
There is simply no way that I could interpret a chord sequence as just a sequence of letter names. Let's look at "Streets of London" again. It starts with the sequence C-G-Am-Em. In my mind I imagine that as "tonic-dominant-submediant-mediant". I don't think in words, of course - I think in sounds. But I'm not trying to imagine what G major sounds like in isolation. I'm imagining what it sounds like relative to C major. If it were written in F major, as F-C-Dm-Am, I'd imagine the same thing. I might not imagine it at the right pitch but I'd get all the intervals right.
It's interesting to note in passing that the chord symbols used in the Baroque era were relative to the key rather than absolute - they'd use "I" for the tonic, "V" for the dominant, "IV" for the subdominant and so on. I think that's a much more intuitive way of notating chords, to be honest, although if you're playing an instrument you have to learn which symbol corresponds to which chord in each key. But it comes with practice.
So that's the way that I interpret and create music, at any rate. It works for me and I couldn't do it any other way.