A shell jacket is usually made up of three components, or layers: an outer shell, a membrane and a lining. The shell is the durable, tightly knitted fabric that gives the jacket its characteristic look and feel, and the membrane is what stops water droplets from penetrating the jacket. If the membrane is breathable, it also lets excess moisture evaporate from your body. The lining in turn protects the membrane from wear and tear and makes it possible to have pockets on the inside of the jacket. In most jackets, the membrane and the outer shell are bonded together whereas the lining consists of a separate, softer fabric; this is called a 2-layer shell jacket. In more technical jackets, all three layers are sometimes bonded together to save bulk and weight. These are called 3-layer jackets and are more common in the outdoor industry.