About clipping masks
A clipping mask is an object whose shape masks other artwork so that only areas that lie within the shape are visible—in effect, clipping the artwork to the shape of the mask. The clipping mask and the objects that are masked are called a clipping set. You can make a clipping set from a selection of two or more objects or from all objects in a group or layer.
Object-level clipping sets are combined as a group in the Layers panel. If you create layer-level clipping sets, the object on top of the layer clips all of the objects below it. All operations that you perform on an object-level clipping set, such as transformations and alignment, are based on the clipping mask’s boundary, not the unmasked boundary. Once you have created an object-level clipping mask, you can only select the clipped content by using the Layers panel, the Direct Selection tool, or by isolating the clipping set.
For a video on using Pathfinder effects and clipping masks and how to import clipping masks into Flash, see www.adobe.com/go/vid0057.
The objects that you mask are moved into the clipping mask’s group in the Layers panel if they don’t already reside there.
Only vector objects can be clipping masks; however, any artwork can be masked.
If you use a layer or group to create a clipping mask, the first object in the layer or group masks everything that is a subset of the layer or group.
Regardless of its previous attributes, a clipping mask changes to an object with no fill or stroke.
Tip: To create a semitransparent mask, use the Transparency panel to create an opacity mask.
Drawing Enhancements Illustrator CS5
A quick tutorial on clipping masks
Hide parts of objects with a clipping mask
Create a clipping mask for a group or layer
Edit a clipping mask
Edit paths within a clipping set
To edit portions of a path that fall outside of the clipping mask, you must first select the specific path within in the clipping mask boundary, and then edit the path.