Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online.Order theory is a branch of mathematics that studies various kinds of binary relations that capture the intuitive notion of ordering, providing a framework for saying when one thing is "less than" or "precedes" another. This article gives a detailed introduction to the field and includes some of the most basic definitions. For a quick lookup of order-theoretic terms, there is also an order theory glossary. A list of order topics collects the various articles in the vicinity of order theory.