Mask layer position in stack

[trailertrash]

I have searched for this and I know it has been asked before, but I just cant get this right. I want to learn masks a bit better and when I try to insert a mask between 2 images so that as the mask passes over the top image it reveals the image below. I had it written down but cant remember the layer order for this. Any help out there please?

Andrew

[davegee]

Something like this?

[trailertrash]

Thanks Dave, thats it..