Adobe Illustrator CS

A clipping mask can be made up of any vector object, including editable text. To create a clipping mask, you select both the path that will be the mask and the art that you want to appear inside of the mask. The path that you want to use as the mask must be the topmost object in your selection. Choose Object > Clipping Mask > Make, and only art that falls within the boundaries of the mask object will remain visible.

Note: A clipping mask uses the vector path as the boundary for the mask, even if the object has an appearance applied to it.

When Illustrator creates a clipping mask, the mask and the art are grouped together, making it easy to move them around as a single unit when you’re using the Selection tool. However, you can use the Direct Selection tool to select and edit just the mask, or just the artwork independently. In truth, because the masked art is still selectable, it can make life difficult when you have a lot of masks in a file, because you can easily select masked artwork when you don’t intend to do so. At the same time, this behavior does make it easy to edit artwork within masks. Getting into the habit of using the Layers palette to lock the art that you aren’t working with for a particular task helps avoid this.