A preface is an introductory piece written by the author of a book. It is distinct from the body and written by an expert in the field, lending legitimacy to the book's subject.... A preface also serves as an outline for the book. Prefaces are found in all kinds of books, including history, science, philosophy, and politics. They often explain what kind of book it is (e.g., a treatise on law) or list other works by the same author. Some prefaces are very short while others can be several pages long. In general, the more prestigious the journal or collection, the longer the preface will be. The term "preface" comes from the French word préface, which means "opening". However, in academic writing, a preface does not always include an opening paragraph; instead, it may serve as an introduction at various points during the text. Generally, there are three types of preface: explanatory, critical, and promotional. An explanatory preface gives information about the book's subject matter or authority that helps readers understand the text.