Even though music is something you listen to and hear, I have always looked for ways of picturing it visually, both for my own understanding and as a way to teach. Below is an analogy (taken from my Planetalk Course) that may help some of you get a grip on how music connects up.

I have often stressed how important it is to know your keys. By that, I mean know which chords belong in each family or key. I guess that’s the first analogy right there. I picture a 'key' as being a family of chords that have arisen out of the major scale. Seven notes --the scale -- give rise to seven chords and together become the key. These seven chords are built simply by picking three alternate notes from the seven notes. Seven starting notes -- seven chords. 

If you're not familiar with this yet, go to Chords - What they are and where they come from.

Let's have a look at this using a real key, for simplicity sake, C. It's nice and uncluttered, with no sharps or flats to cloud the issue. 

The scale reads C - D - E - F - G - A - B 

The chords which arise out of that scale are: C, Dm, Em, F, G, Am, and B half diminished (and back to C). the different flavors arise from the irregularity of the intervals in the scale ... Tone Tone Semitone Tone Tone Tone semitone.

These chords can also be represented generically using Roman numerals.

I - ii - iii - IV - V - vi - vii. Notice how the upper case represent the 3 Major chords, lower case represent the 3 minors and the half-diminished (otherwise known as 'minor7flat5').

These Roman numerals can now apply to all 12 keys. They are referred to in spoken language as the 'One Chord', the 'Two Chord', the 'Three Chord', etc. You might hear in a recording studio, for example, the producer saying to the musicians "Bring it all down to a whisper when you hit the Three Chord at bar 12" or "Watch out for the stop when we get to the Four Chord". The players will automatically know which chords the instructions refer to, because they know the key the tune is being played in. If the singer decides to use another key that is more suitable for her voice, the Roman numerals will remain unchanged, the instructions will still apply and the musicians will automatically know what the new chords are. (because the know their keys.)

So, how to visualize all of this? When I learned about all this, I found myself imagining the key as an apartment, or a 'flat' as they say in the UK. The apartment has seven rooms, just as a key has seven chords, and the rooms all have their permanent place in the layout and a specific function. Like so:

You can see that I've made the large rooms the three major chords ... major chords - major rooms. Let's call the I chord the living room; the IV chord the main bedroom; the V chord the office. It doesn't really matter what you call them, the main thing to remember is that they are fixed in the layout and they have a certain function within the apartment.

We can then view the ii chord as the kid's bedroom, the vi chord as the kitchen, the iii chord as the bathroom and the vii chord as the laundry ... or whatever. All rooms connect and are laid out in a fixed way, and of course you have no trouble finding your way around the apartment once you learn the layout.

That's one key.

All music is a twelve-story apartment block, twelve floors of identical layout, one floor for each of the twelve notes of the chromatic scale. The Music Building. 

So, each key consists of seven chords, all in the same fixed relationship with each other. If you know one, you know 'em all. The I chord will always be the living room, the vi chord, always the kitchen, etc., etc. Same function. 

If some of you are thinking, "Seven times twelve!?!? That's 94 chords! How can I remember all those chords!?!?", relax. There aren't that many. 'Am', as an example, shows up on three floors -- once as the vi chord of the key of C, once as the ii chord of the key of G, again as the iii chord of the key of F. So it's the kitchen in one, the kid's bedroom in another, the bathroom in the next. Same chord, different function. The same goes for the other chords: C for example, is the living room of 'C', the master bedroom of 'G' and the office of 'F'.

As confusing as all this sounds when you're starting out, it becomes second nature quite quickly if you apply yourself. It really is essential to know this stuff if you want to become fluent. I find it comforting to be able to picture music as being confined within this twelve-story apartment block, especially as all floors are identical. I used to think it was endlessly complicated, but actually, it all lives in this one building. All scales, notes, chords, riffs, styles, even the minor keys ... it's all inside that building. (I have debated this point at length on other forums, but I stand by my statement: all music is somewhere inside this Music Building ... it has to be.)

The difficulty really comes from the fact that we have twelve floors and only seven letters by which to name them, so we have to add "sharp" and "flat" to some to make up the shortfall. But there is nothing different about a sharp or flat. Just another of the twelve notes/keys/apartments.

When a piece of music changes key, you move from one floor to another.

I hope this helps you get some kind of a grip on the way music is structured.