Clipping and Masking
These two features are both employed in the same way: put the object that will serve as clip (or mask) above the object that you want to clip (or mask). Then select both objects, and useor . The top object will then become invisible and work to hide parts of the bottom object.
Clips and masks give different results. A clip allows you to hide all parts of the bottom object that are outside the clip object. The colors of a mask object affect the opacity of the bottom object. The darker and the more transparent the mask, the more transparent the masked object will become. A white mask works just like a clip.