A layer mask controls the visibility of the contents of a single layer, but a Clipping Mask controls the visibility of the contents of multiple layers. If you need to establish a common edge for multiple layers, this is the easiest way to do so. Using a Clipping Mask requires a bit of upside-down thinking, though. We’re accustomed to a mask controlling what’s underneath it, based on real-world examples such as the mat around a framed painting. But a Clipping Masks falls below the content it masks (Figure 9.7). The color of the painted area of the mask layer isn’t important; the transparency or opacity of each pixel in the mask layer governs the transparency and opacity of the layers above it in the group.