[AI CS4 Mac] How to Create a Layer Clipping Mask?

Clipping masks work, but they can be a little clunky (as you might tell from their weird way of being set up). You might find opacity masks more flexible. An opacity mask is basically a group attached to a layer, in which you store paths that represent holes in the mask. These holes have an opacity value for how much shows through. So anything in the mask group is a hole (there can be many) and anything in the layer (as normal) shows through those holes. Setting it up is simpler but it depends on the kind of clipping you're trying to do which is easier. We find it easier when doing knockouts for labels on lines, but clipping paths are also possible, just a little more annoying.