A paperback or soft cover book consists of various sections with a cover usually made with heavier paper, glued together at the spine with a strong flexible glue; this is sometimes called perfect binding. In perfect binding, the sections are rough-cut in the back as to make them absorb the hot glue. The other three sides are then trimmed. This is what allows the magazine or paperback book to be opened. Mass market paperbacks (pulp paperbacks) are small (16mo size), cheaply made and often fall apart after much handling or several years. Trade paperbacks are more sturdily made, usually larger, and more expensive.