Masks and matte

Could someone give me a simple explanation on how I could take a black and white image/ Video, i.e., a solid white cube on a black background, and turn it into a mask so that it edits the alpha layer of another clip? A node setup would work fine.

:wink:

http://wiki.blender.org/index.php/Doc:Manual/Sequencer/Effects#Alpha_Over.2C_Under.2C_and_Over_Drop_Built-in_Effects

I meant in the node editor. I want to make it so that the white area shows one clip and the black area shows the other.

Oh! I see! Thanx!