The Britannica is the oldest English-language encyclopaedia still in production. It was first published between 1768 and 1771 in the Scottish capital of Edinburgh, as three volumes. The encyclopaedia grew in size: the second edition was 10 volumes, and by its fourth edition (1801–1810) it had expanded to 20 volumes. Its rising stature as a scholarly work helped recruit eminent contributors, and the 9th (1875–1889) and 11th editions (1911) are landmark encyclopaedias for scholarship and literary style. Beginning with the 11th edition and following its acquisition by an American firm, the Britannica shortened and simplified articles to broaden its appeal to the North American market. In 1933, the Britannica became the first encyclopaedia to adopt "continuous revision", in which the encyclopaedia is continually reprinted, with every article updated on a schedule. In March 2012, Encyclopædia Britannica, Inc. announced it would no longer publish printed editions, and would focus instead on Encyclopædia Britannica Online.
The 15th edition has a three-part structure: a 12-volume Micropædia of short articles (generally fewer than 750 words), a 17-volume Macropædia of long articles (two to 310 pages), and a single Propædia volume to give a hierarchical outline of knowledge. The Micropædia is meant for quick fact-checking and as a guide to the Macropædia; readers are advised to study the Propædia outline to understand a subject's context and to find more detailed articles. Over 70 years, the size of the Britannica has remained steady, with about 40 million words on half a million topics. Though published in the United States since 1901, the Britannica has for the most part maintained British English spelling.
Since 1985, the Britannica has had four parts: the Micropædia, the Macropædia, the Propædia, and a two-volume index. The Britannica's articles are found in the Micro- and Macropædia, which encompass 12 and 17 volumes, respectively, each volume having roughly one thousand pages. The 2007 Macropædia has 699 in-depth articles, ranging in length from 2 to 310 pages and having references and named contributors. In contrast, the 2007 Micropædia has roughly 65,000 articles, the vast majority (about 97%) of which contain fewer than 750 words, no references, and no named contributors. The Micropædia articles are intended for quick fact-checking and to help in finding more thorough information in the Macropædia. The Macropædia articles are meant both as authoritative, well-written articles on their subjects and as storehouses of information not covered elsewhere. The longest article (310 pages) is on the United States, and resulted from the merger of the articles on the individual states. The 2013 edition of Britannica contained approximately forty thousand articles.
Information can be found in the Britannica by following the cross-references in the Micropædia and Macropædia; however, these are sparse, averaging one cross-reference per page. Hence, readers are recommended to consult instead the alphabetical index or the Propædia, which organizes the Britannica's contents by topic.
The core of the Propædia is its "Outline of Knowledge", which aims to provide a logical framework for all human knowledge. Accordingly, the Outline is consulted by the Britannica's editors to decide which articles should be included in the Micro- and Macropædia. The Outline is also intended to be a study guide, to put subjects in their proper perspective, and to suggest a series of Britannica articles for the student wishing to learn a topic in depth. However, libraries have found that it is scarcely used, and reviewers have recommended that it be dropped from the encyclopaedia. The Propædia also has color transparencies of human anatomy and several appendices listing the staff members, advisors, and contributors to all three parts of the Britannica.