TRANSPARENCY IS A BIT OF A GRAPHIC ARTS PARADOX, because the way you know it's working is when you can't see it. That's because transparency does its job not by occupying space, but by making space for something else. In Photoshop, you can apply transparency to an entire document so that it has a transparent background when you use the Photoshop document in a page-layout program such as Adobe InDesign, or on a Web page. Transparency also has an important use within a single Photoshop document: When you create a composition using multiple layers, transparency is what lets you see the layers behind the topmost layer. If transparency wasn't available, the entire top layer would be an opaque rectangle, and you wouldn't be able to see anything behind it.
Transparency isn't just for removing backgrounds or creating collages of different images. You can also use transparency to combine different versions of the same images, such as different exposures of a shot.
When you use transparency in this way, you combine images to show the best and hide the worst of each image. In this chapter I cover the essentials of transparency so that you can get the results you want as quickly as possible.