Indexes can be searched by subject or author, and in many cases by key word, periodical title, date, language, etc.
Some indexes have the electronic full text of some or all of the articles they index.
To find a periodical article in a library, you must know its name, what periodical it was published in, and the date, volume, and issue number of that periodical. This information comes in a statement called a citation. Citations are found by using periodical indexes. Indexes are listings of all the articles in a given list of periodicals. An index can be searched by subject, by author, or, if computerized, by any word in the citation or abstract.