OK, so I bought a 3rd Party lower third video for a project I was working on.  I guess I expected the video to have a green background and I would just use ChromaKey to make the lower third overlay the primary video.  But once I downloaded it, I noticed the video background just appeared black when I watched it.

So I dropped the lower third on the lower video track and 'magically' it just worked as I needed.  

How are these type of videos made?  Is there some way to make the background be "invisible"

Thanks,

Doug