Form is the key that unlocks the musical map of a song or a musical compostition. Like a house has rooms, so a song has rooms or sections.

Chord progressions come in sections, like one room in a house. You can put several different rooms together to make a big house, or you can live in a one room house. Just like people. In most 3rd world countries people live in one room houses -- which means, of course, that much of the world lives in one-room houses.

Those of us who live in the West generally live in multi-room houses.

But there are also musical houses -- we call them songs -- that are built out of several different rooms -- several different chord progressions. Some of them, like mansions and castles, go on and on and get quite involved.

But most songs are like many modest houses -- they have 2 or 3 rooms, sometimes 4 -- built using 2 or 3 or 4 different chord progressions.

Each "room" in a musical house is called a theme, or a "motif". The first theme is always called "A". The next theme is called "B", the next theme is called "C", and so on. Most songs only have 2 or 3 themes, but these themes often repeat.

For example, let's say we have a chord progression that goes like this:

C Am7 Dm7 G7

...and then it repeats those same 4 chords...

and then we have another chord progression that goes like this: Gm7 C7 F Fm7 Bb7 Eb G7

...and then the first chord progression is used again as the song ends.

This song would have a musical form of A, A, B, A -- main theme, repeat of main theme, contrasting theme, main theme.

If a song went like this:

Theme contrast theme