Masking: Rounded Corners in Fireworks
Tutorial by Patricia Janann Nicholson

Masking is basically seeing one object through another. Sort of like a window. Effects can then be added to the mask or "window" which then change how the underlying image appears. In this example we will create a rounded corners on the top two corners of this photo. The window or mask will be a rectangle with rounded corners that is higher than the photo and then positioned so that only the top two corners are on the canvas. When the rectangle is masked to the image we only see the image through the rectangle and outside of the rectangle we just see the canvas where the image used to be.