An index ( plural: usually indexes, more rarely indices; see below) is a list of words or phrases ('headings') and associated pointers ('locators') to where useful material relating to that heading can be found in a document or collection of documents. Examples are an index in the back matter of a book and an index that serves as a library catalog. An index is not the same thing as a table of contents. The table of contents (or contents page), which is found in the front matter of a book, lists parts, sections, and chapters of the book and their corresponding page numbers. This is for navigational purposes, just as the index is, but the index has more to do with subjects. The index found in a book is a list of the topics, names and places mentioned in it, together with the page numbers where they can be found. The index is usually found at the back of a book. Only those words that have relevance to the book's main themes are chosen for the index.