Principal types of binding are padding, perfect, spiral, comb, sewn, clasp, disc, and pressure, some of which can be combined. Binding methods can affect whether a notebook can lie flat when open and whether the pages are likely to remain attached. The cover material is usually distinct from the writing surface material, more durable, more decorative, and more firmly attached. It also is stiffer than the pages, even taken together. Cover materials should not contribute to damage or discomfort.