DirectX confusion?

Guys, help me comprehends this whole thing with DirectX. As far as I understand, in the past each new DirectX version would replace the old version, but retain backward compatibility. But now with Vista and DirectX 10, I understand there needs to be both DX9 and DX10 on your system, and they are independent of each other? And how that Win7 is out and comes with DX11, is it going to need 9, 10 and 11?

For instance, when I first installed vista, it came with DX10, and if my memory serves me right, DX9 games would not work at all until I installed DX9. Does that mean that there are two different versions of DirectX on the system? When I run dxdiag it only shows DX10.

I know in the past each DirectX version replaced the previous version, but remained backward compatible. For instance DX3 games will run on DX9 without requiring you to install DX3.

So if there are two or three different version of DX on Vista and Win 7, is there a way to run dxdiag for DX9?

Hmmm,

DirectX confusion? - Rage3D Discussion Area

and to answer your question there from today,

why wouldn't a DX9 game run on Vista out of the box, until DX9 is installed?

because the required DX9 files aren't there on a clean Vista or 7 install (and there is quite a few of them); either until you install them using teh DX update (which was linked to in the first response there), or they get installed at the time that you install the game.