Lining The lining is the inside of the bag or the part that you actually lay on. Like the shell, linings come in a number of materials. Lining materials can include silk, taffeta, nylon, polyester, cotton, or fleece. When selecting a lining consider your personal comforts. Then fleece or nylon may not be the best choices.
Silk is wonderful but very expensive and rips easily.
Cotton is comfortable and inexpensive, but it is not very durable and Fill Fill is what the sleeping bag is stuffed with. The fill is what keeps you warm. Fill comes in basically two types of materials, natural down, typically goose feathers, and synthetic or man made fibers. Pound for pound nothing will keep you warmer than down. However down is more expensive than man made material and if it gets damp, looses almost all of its ability to keep you warm.